- About this Journal
- Abstracting and Indexing
- Aims and Scope
- Article Processing Charges
- Articles in Press
- Author Guidelines
- Bibliographic Information
- Citations to this Journal
- Contact Information
- Editorial Board
- Editorial Workflow
- Free eTOC Alerts
- Publication Ethics
- Submit a Manuscript
- Subscription Information
- Table of Contents
Volume 2013 (2013), Article ID 457698, 33 pages
Sustaining Cavity-Using Species: Patterns of Cavity Use and Implications to Forest Management
Forest Sciences Department, University of British Columbia, 3041-2424 Main Mall, Vancouver, BC, Canada V6T 1Z4
Received 12 September 2012; Accepted 18 October 2012
Academic Editors: F. Castedo-Dorado, T. S. Fredericksen, H. Nahrung, and J. F. Negron
Copyright © 2013 Fred L. Bunnell. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
Many bird and mammal species rely on cavities in trees to rear their young or roost. Favourable cavity sites are usually created by fungi, so they are more common in older, dying trees that are incompatible with intensive fiber production. Forestry has reduced amounts of such trees to the extent that many cavity-using vertebrates are now designated “at risk.” The simple model of cavity use presented helps unite research findings, explain patterns of use, and clarify trade-offs that can, or cannot, be made in snag management. Predictions generated are tested using data from over 300 studies. Implications to forest management are derived from the tests, including the following: ensure sustained provision of dying and dead trees, retain both conifers and hardwoods and a range of size and age classes, sustain a range of decay classes, ensure that some large trees or snags are retained, promote both aggregated and dispersed retention of dead and dying trees, meet dead wood requirements for larger species where intensive fibre production is not emphasized, do not do the same thing everywhere, and limit salvage logging after tree mortality. The paper focuses on species breeding in the Pacific Northwest, but draws on data from throughout those species’ ranges.
Most cavities in trees begin with fungi. Because trees resist decay, it takes time for fungi to soften wood enough that cavity excavation by birds is possible. By that time, trees are often old and beginning to die. Old and dying trees reduce economic efficiencies within managed forests, so for decades we have sought to remove them. Our actions were successful, and cavity sites have been much reduced [1–3]. I focus on the Pacific Northwest of North America (PNW), here defined as Alaska, Yukon Territory, Alberta, British Columbia, Washington, Oregon, Idaho, Montana and northern Nevada, and California. Of the 67 vertebrate species commonly using cavities in the PNW, 20 (30%) are designated “at risk” or “potentially at risk.” Where forestry has been practiced longer, the proportion of cavity users among forest-dwelling vertebrates designated “at risk” is higher .
I review kinds of cavity use, present a general framework of cavity use in the PNW, review key factors influencing cavity use, and interpret those in terms of management implications. Focus is on primary excavators, but all birds and mammals commonly using cavities are included. Summary tables and figures highlight regional differences: coastal forests (under maritime influence), subboreal plus boreal forest, and inland (all other forests). Common and scientific names follow the American Ornithologists’ Union (birds) and British Columbia Conservation Data Centre (mammals).
2. Kinds of Cavity Use
Two broad groups of cavity users are distinguished: primary excavators, such as woodpeckers, that excavate their own cavities and secondary cavity users, such as bufflehead ducks (Bucephala albeola), tree swallows (Tachycineta bicolor), and northern flying squirrels (Glaucomys sabrinus), that use holes excavated by primary cavity excavators. Secondary users include species that seek a particular, often uncommon, form of natural cavity, such as brown creepers (Certhia americana), Vaux’s swifts (Chaetura vauxi), and several bats plus opportunistic species that use cavities 50% or more of the time in some areas but not everywhere, such as black bears (Ursus americanus), porcupine (Erethizon dorsatum), and some bat and rodent species. In analyses following, such unconventional and opportunistic users are grouped with other secondary cavity users.
Primary excavators show different abilities to excavate. Two groups are recognized: species that forage by drilling, boring, or hammering into wood or soil and species that probe or glean bark, branches, and leaves to acquire prey. The former group is termed strong excavators. It includes most woodpeckers, sapsuckers, and the northern flicker (Colaptes auratus). Strong excavators are typically well-adapted for creating holes in trees and have reinforced skulls and ribs and chisel-like beaks . Weak excavators include chickadees, nuthatches, and those woodpeckers that forage primarily by probing and gleaning, extracting seeds, or capturing insects in flight (e.g., acorn (Melanerpes formicivorus), downy (Picoides pubescens), Lewis’s (M. lewis), Nuttall’s (P. nuttalli), and white-headed (P. albolarvatus) woodpeckers). Despite using cavities to nest, weak excavators are less well adapted to excavation than are species that drill into wood to forage, so often use cavities initiated by strong excavators. Strong excavators are generally large birds; weak excavators are mostly smaller species.
The larger species of strong primary excavators can act as keystone species, providing nest, den, and roost sites for other cavity-using species. If their requirements are lacking, secondary cavity users may be lost [7–11]. Similarly, sapsucker foraging activity creates feeding opportunities for many other species. At least 23 bird species, 6 mammal species, and numerous arthropods (9 orders and 22 families) have been reported feeding at sapsucker holes [12–14]. Woodpeckers also can sometimes constrain the abundance of forest “pest” insects [15, 16]. Loss of strong excavators would seriously disrupt forest ecosystems.
In the PNW, 67 vertebrate species use cavities more than 50% of the time, either generally or regionally; more species are opportunistic in their use of tree cavities. A small component of strong, primary excavators creates cavity sites for many more species (Figure 1). There are 22 primary cavity excavators and 45 secondary cavity users relying on hollows or cavities, not all of these excavated by birds. Only 9 species are strong excavators, affirming the role of strong excavators as keystone species. Most secondary users rely on holes excavated by primary excavators. The proportion of nest sites of secondary users excavated by other species ranged from 89 to 100% with one exception ; neither of two flammulated owl (Otus flammeolus) nest sites was excavated.
Birds (48 species) and bats (11 species) represent 88% of species consistently or commonly using cavities. More bird than mammal species use cavities; mammals using cavities or hollows range in size from bats to grizzly bears (Ursus arctos). Other than for some bats and squirrels, mammal use of cavities is more opportunistic than it is for birds. Amphibians and reptiles also use cavities in snags and stumps opportunistically [17–19].
Larger snags provide more room and are longer lived, so provide greater opportunities for cavity use. The number of cavity-using species thus decreases with decreasing diameter of the dominant tree species. In the north, where trees are small (Spruce Willow Birch of Figure 1), the numbers of cavity users is much reduced.
Figure 1 excludes opportunistic cavity users that are included with secondary users in analyses following. Many of these are smaller birds and mammals that do not require cavities but will use them; some larger species (e.g., great horned owl, Bubo virginianus, and porcupine) also are opportunistic. In any forest type there also are a few individualist species that seek particular cavities not excavated by birds. Vaux’s swifts, for example, nest and roost in hollow snags large enough that they can circle into and out of them . Brown creepers, like some amphibians and reptiles, often nest or seek cover under slabs of loose bark . Some cavity users (American martens, Martes americana; fishers, M. pennanti; black bears) are too large to rely on cavities excavated by birds. They rely on cavities formed by decay or fire. Such cavities are becoming uncommon, because old, large trees are increasingly uncommon.
3. Biology of Cavity Use and Associated Predictions
Hairy woodpeckers (Picoides villosus) need about 20 days to excavate their nest , as do yellow-bellied sapsuckers (Sphyrapicus varius) . That is 20 days of hammering bill and head against wood in a small space, at rates of 100 to 300 times per minute . There has to be some benefit to this behavior, and there is: cavity-using species benefit from the protection cavities, provide their young from predators, and shelter from weather in the nest, den, or roost. Because of their high-energy demands and sensitivity to temperature, bats often select cavities with favourable microclimates [25, 26]. For birds, the ideal cavity nest is located where the outer wood is hard to prevent predators from tearing open the nest, but the inner wood is soft to allow easy excavation. Soft inner wood may be required, due to the difficulty of chiseling wood in an enclosed space. Heart rots, preferably localized, are thus of prime importance. Generally, all primary excavators seek decayed heartwood [27–31]. Stronger excavators will chisel through sound wood to reach decay. For weak primary cavity nesters, harder outer wood is desirable to reduce predation, but these species are not adapted to excavating hard wood. They must either compromise with softened sapwood, or find existing holes in hard outer wood, such as those at dead branch stubs. The ideal cavity also should be high up a tree to avoid ground-dwelling nest predators, such as weasels (Mustela) and rodents.
The bole or branch, where the nest is located, must be wide enough to accommodate the animal and its eggs or young. Size of the cavity can affect productivity; several cavity-using species have been found to be more productive in larger nest boxes [32–35]. Together, these two factors (protection from predators and size) promote a preference for large diameter trees in which the bole is a suitable width at a greater height. When trees are sound, greater size also reduces the danger of the tree or snag breaking. The importance of softened heartwood also promotes preference for larger trees, because larger trees are usually older trees that have had time to experience heart rot. The search for decayed wood, appropriate stem diameter at the nest, and height above the ground explain many of the preferences documented for primary excavators.
Unlike primary excavators, excavating a cavity is not part of the breeding behaviour of secondary cavity users. With regard to cavities, secondary cavity users “take what they can get” with certain limitations. The most obvious limitation is that the cavity must be big enough. Mountain bluebirds (Sialia currucoides), for example, rely primarily on nests made by flickers, rather than those of the smaller woodpeckers or sapsuckers . The nest also needs to be near the appropriate foraging habitat, which is not forest for many secondary cavity users (e.g., cavity-nesting ducks, swallows, bluebirds, American kestrels, Falco sparverius). Whereas primary excavators usually make or modify a new cavity each year, secondary users will return to the same cavity as long as it remains stable . Because secondary users are relatively abundant compared to the primary excavators, competition for cavities can be intense [37, 38]. Where opportunities exist (e.g., holes at shed branches, stem breakage), secondary users seek more stable, living trees, but many end up using preexcavated holes in dead trees.
The preceding statement on the biology of cavity use provides a simple model or framework to unite research findings, explain patterns of observed use, and clarify trade-offs that can, or cannot, be made in snag management. The framework also generates expectations or predictions that can guide the questions posed in synthesis. Four broad predictions and associated corollaries were tested using attributes commonly employed to describe trees or snags used as cavity sites (tree species, size of tree, decay state, and snag density).(1)Ability to excavate wood permits greater selectivity, with the range of sites used becoming broader from strong excavators through weak excavators to secondary users. Where possible, secondary users will use live trees.(2)Strong excavators will select trees with less visible signs of decay than will weak excavators. Intense competition among secondary users will relegate most to use dead trees, where existing cavities are more common, rather than more stable, live trees. (3)To attain desired protection, selection of tree species will reflect the tree’s ability to compartmentalize decay. Deciduous trees more often contain internal rot surrounded by a sound outer shell than do conifers, so will be sought preferentially. (4)The search for sufficient room and protection from predators will produce selection for the largest suitable cavity site available, as high above ground as possible. Strong excavators will enact selection more effectively.
The energy demands of excavation make softened wood or decay a dominating factor in selection of cavity sites. I review how decay is initiated, then examine patterns of use by primary excavators of cavities and secondary users.
4.1. Natural Disturbance and Decay
How a tree dies influences snag longevity and onset of decay. Research has identified tree species, tree size, decay stage, crown scorch, and stand density as major factors determining snag longevity [39–43]. Natural fire regimes influence the kinds and amounts of dead wood present . The role of fire is apparent within the 12 broad forest types of British Columbia. Within forest types, the proportion of species using cavity sites decreased significantly with increasing fire size and intensity; species richness of cavity users decreased significantly as fire-return interval lengthened, and snags were created less frequently [45, 46]. Generally, snags created by fire fall sooner than do other snags. The species of snag has less effect on snag longevity than the method of creation and tree diameter . Trees of larger diameter remain standing longer regardless of source of mortality, and pines (Pinus) stand longer than firs (Abies) . Pinus ponderosa killed by fire remained standing longer than those killed by bark beetles [47–49]. Fire can encourage beetle attack by weakening the tree. Beetle-killed trees are more attractive to cavity nesters that tend to excavate nest sites in trees on which they have foraged .
Wounds influence the suitability of snags by encouraging different patterns of decay. Tops broken by wind or snow encourage nesting by species from black-capped chickadees (Poecile atricapillus) to spotted owls (Strix occidentalis). In conifers, decay occurs primarily in older live trees with defects in the outer sapwood, such as broken tops. Heart rots can create the hollow trees and hollow logs sought by Vaux’s swift, American marten, fisher, and black bears. Large cavities result from a fire or a living tree’s defense against fungi. A tree without decay that succumbs to windthrow will never become hollow. In most most temperate hardwoods, heart rots may not require a wound to enter, occur earlier than in conifers, and are common among otherwise healthy, young trees. In some species, sapwood decays at the same time as heartwood (e.g., Douglas-fir, Pseudotsuga menziesii, on wet sites), while in others, sapwood may not begin to soften until decades after the tree has died, and the centre has rotted (e.g., cottonwood and poplar, Populus balsamifera sbsp).
Because different agents of wounding or mortality create differ forms of decay and snags, local natural disturbances help explain the abundance of particular cavity-using species in an area. Usually, the forms of mortality are too varied to have predictive utility over large regions, but successful attempts to reduce specific forms of mortality can impact the fauna. Fire suppression in lodgepole pine (Pinus contorta) helped create the greatest insect mortality recorded in North America, with subsequent major impacts on bird species . Similarly, harvest of potential nest trees before heart rot became prevalent would seriously impact Vaux’s swifts. More generally, natural disturbance regimes can provide broad guidelines to the provision of cavity sites by revealing regional patterns in the onset of decay.
4.2. Species Patterns with Decay
Many decay agents are not readily visible in the field, so researchers classify decay of live trees and snags using a set of visual classes. Most researchers in the PNW follow the classification of . Wood in classes 6 through 9 of that classification is too soft to be used by cavity nesters; the five harder classes are used here. Those classes are most simply defined as 1 (live), 2 (declining), 3 (dead, bark intact), 4 (dead, loose bark), and 5 (dead, no bark). Hardness of living trees and their resistance to excavation as determined mechanically is not consistently related to visual signs of decay , but broad patterns with visual signs of decay are evident (Figure 2).
In North America, primary excavators, both weak and strong, often create 80% or more of their nest sites, relying on natural cavities for the rest [7, 9, 11, 53, 54]. Prediction 1 that strong excavators are able to select cavities in less decayed wood than can weak excavators is met (Figure 2).
When nesting in trembling aspen (Populus tremuloides), the weighted mean decay classes for strong and weak excavators are 1.9 (live) and 3.8 (dead), respectively. Visual estimates indicate more advanced decay when nesting in conifers: 2.8 and 4.8 for strong and weak excavators, respectively. Sapwood of aspen remains firm for years after tree death . Even in forests dominated by conifers, large strong excavators (e.g., yellow-bellied sapsucker, Sphyrapicus varius, and pileated woodpecker, Dryocopus pileatus) preferentially nest in trembling aspen having decayed heartwood surrounded by sound sapwood [29, 55]. They use live and dead aspen relatively indiscriminately. Most pileated woodpecker nests in aspen are in live trees , affirming their ability to excavate live wood and the value of a sound sapwood shell. Conversely, weak excavators, such as boreal chickadee (Poecile hudsonica), nuthatches (Sitta spp.), downy, and acorn woodpeckers, do not excavate through hard sapwood shells. They seek dead trees or dead portions of living trees [46, 69, 70]. Among secondary cavity users, both birds and mammals tend to use less advanced stages of decay in hardwoods than in conifers (Figure 2). For waterfowl, too few decay classes for nest sites in conifers have been reported to permit comparisons with hardwood use.
More studies report whether a nest tree is living or dead than report decay class (Figure 3). Two points are clear. First, tree death provides nesting opportunities; 17 of 22 species in Figure 3 rely primarily on dead trees as nesting sites (decay class 3 and greater). The large portion of dead trees among cavity sites of all excavators emphasizes the importance of rot in determining nesting opportunities. In montane aspen stands, the number of nest holes was significantly and positively correlated with the percent of trees infected by Fomes (Phellinus) . In coastal forests of the PNW, the marked preference cavity nesters show for western hemlock (Tsuga heterophylla) over western red cedar (Thuja plicata) has been attributed to the greater frequency of rot in hemlock [27, 72]. Woodpeckers may nest low in decay-resistant trees where butt rots are more prevalent than stem or top rots [27, 73]. In short, excavators create cavities where decay permits, and those trees are often dead.
A second point is that weak and strong excavators use trees differently. Across all 9 species of strong excavators in Figure 3, the interspecific average use of dead trees is 53.4%. Of 4403 nest trees used by strong excavators, 2229 (51.8%) were dead. Eleven of 13 species of weak excavators relied primarily on dead trees (Figure 3), even when more cavity-prone hardwoods comprised 80–100% of the nest sites. The acorn woodpecker is an exception that nests in live trees but seeks out dead limbs for cavity sites . Excluding the acorn woodpecker, weak excavators located 70% (1831/2617) of their cavities in dead trees; the interspecific average across species was 74.8%. The abundance of suitable dead trees influences both groups, but weak excavators are less able to nest in living trees, and they are the larger group of species (Figure 1). Some small, weak excavators exploit advanced decay, even though it provides less protection. For example, 39 of 94 cavities excavated by chestnut-backed chickadees (Poecile rufescens) were close to the ground in rotting stumps .
The period that a snag remains firm enough to provide useful protection is shorter than the life of the snag. Analysis of preference for decay classes by comparing use to availability  showed the most strongly preferred decay classes were classes 3 and 4 (recently dead trees). How long a tree remains in these decay classes depends on cause of mortality, tree species, and site. Pileated woodpeckers used cavities in ponderosa pine (Pinus ponderosa) for 3–8 years after the trees were killed by fire .
There are many more secondary users of cavities than primary cavity excavators (Figure 1). Dead trees provide most opportunities for secondary cavity users, whether cavities have been excavated by primary excavators or formed otherwise (Figure 4). The search for live trees and greater permanence is apparent as lower means in decay classes in Figure 2 (cavity sites in decay classes 1 and 2 are sought where available), but prior excavations and other natural cavities are more common in trees already dead (Figure 4).
Mammals denning in trees do not excavate cavities, so are more dependent upon natural cavities and rot patterns than are many birds. However, because competition for cavity sites is intense among secondary cavity-nesting birds, there is little difference in relative use of dead wood between them and mammals (Figure 4). The mean percent use of dead trees across mammal species is 63.0%, it is 60.4% for secondary cavity-nesting birds. Bats are least able to modify an existing cavity and show the strongest selection for dead trees (74.9% across the seven species). Other, larger mammals are more capable of modifying and enlarging an existing cavity in living trees so are less restricted to dead trees (38.4% across 6 species). These larger species require hollows too large to be created by birds.
Decay can create hollow, living trees, and ultimately, hollow logs through heart rot and loss of heartwood residues . Less than 40% of denning trees of northern flying squirrels, American marten, American fisher, and black bears were dead, indicating the importance of sustaining older trees with large rot pockets (Figure 4). For example, most black bear dens recorded from coastal forests of the Pacific Northwest are associated with wooden structures, including trees, logs, and stumps [134, 157]. Den sites in southern, inland forests also are commonly in trees . In dry inland forest, 20 of 23 den sites of radio-fitted black bears were associated with trees. Ten of these were entered through the treetop into hollow centres created by fungi . Even coastal grizzly bears sometimes use hollows in living conifers or stumps .
Some birds also use hollow trees. All 60 Vaux’s swift nests and roosts located in northeastern Oregon were in live or dead grand firs (Abies grandis) with hollow interiors . Amphibians and reptiles use cavities rarely and opportunistically [18, 160]. Such use tends to occur in snags of decay class 3 and 4, when bark is loose and sloughing . Their use of dying and dead trees is thus similar to that of the brown creeper, whose nests are almost always between the trunk and a loose piece of bark on a large, typically dead or dying tree .
Predictions 1 and 2 are met. Because strong excavators have more options, their cavity sites are more closely associated with more stable and preferable sites in live and recently dead trees (Figure 2). Decay classes used by weak excavators tend to be more advanced. The mean decay class of trees used by secondary users is relatively low (Figure 2), indicating a preference for living trees, but most cavity sites are found in dead trees (Figure 4).
5. Tree Species
The ideal cavity is in a stable, living tree. How a tree compartmentalizes fungal attack determines the likelihood of an ideal cavity. Broadleaved, deciduous tree species commonly isolate decay-softened wood inside a hard shell. When assessed by visual decay classes, broadleaved trees harbouring internal decay more often appear healthy (e.g., fully leaved) than do decayed conifers. Broadleaved tree species are conventionally termed “hardwoods.” For a cavity-excavating bird, however, they are “softer” or better sites for excavation than are conifers (conventionally termed “softwoods”). The intact, outer sapwood shell provides stability and structure, with decayed wood a short distance beyond the shell. Conifers, however, often remain standing longer after death than do most hardwoods (large cottonwoods may be an exception). We thus expect (prediction 3) selection to be for live hardwoods when available and towards dead conifers when hardwoods are not available, with foraging preferences sometimes dominating. That expectation is met. Cavity nesters often chose hardwoods for 80–95% of their nest sites even where hardwoods comprised only 5–15% of the available tree stems [29, 53, 55, 162]. For bird species in Figure 2, conifer nest trees tended to exhibit more advanced decay by visual classes than did hardwood nest trees. Most conifers were dead.
Weak excavators are more often limited to dead trees (Figure 3), so availability of living hardwoods has greater influence on cavity site selection by strong excavators. Across 9 species of strong excavators, the percent of dead trees used declined significantly, as the percent of hardwood trees used increased () . The relationship accounts for much variability among the 46 studies aggregated in Figure 5. For example, in a large sample for pileated woodpeckers ( nests) all available nest trees were conifers, and 99% of the nests were in dead trees . Conversely, where hardwoods were available but scarce (<10% of stems), 26 of 27 nest trees were in trembling aspen, more than 85% of them living . Similarly, northern flicker selected snags for 33% of their nest sites when nesting in hardwoods (), increasing to 65% when nesting in conifers () .
Selection for specific tree species as cavity sites has been evaluated for studies reporting both use and availability . In the PNW, tree species selected disproportionately to their availability include trembling aspen, western larch (Larix occidentalis), and ponderosa pine. Trembling aspen is especially favoured. Several bat species also prefer to roost in aspen, probably because aspen is cavity prone [25, 60, 147]. In coastal forests, where aspen is lacking, primary excavators favour bigleaf maple (Acer macrophyllum) and red alder (Alnus rubra) disproportionately. Generally, lodgepole pine and the true firs (Abies) appear least favoured as cavity nesting sites . However, a bird must feed itself before it can nest. Where black-backed (Picoides arcticus) and American three-toed (Picoides dorsalis) woodpeckers concentrate their foraging in beetle-ridden lodgepole pine, they also concentrate their nesting in lodgepole pine [164, 186]. In the absence of recent fires and beetle-infested trees, three-toed woodpeckers’ primary habitat is mature or old-growth coniferous forests with an abundance of insect-infested snags or dying trees . In old-growth Engelmann spruce (Picea engelmannii)-Subalpine fir (Abies lasiocarpa) forests all nests of three-toed woodpeckers were in Subalpine fir .
Although hardwoods are sought when available, use of hardwoods often is flexible. In the southern interior of British Columbia, all 243 nests of primary excavators found occurred in hardwoods, although hardwoods covered only 5% of the study area dominated by Douglas-fir . In western Oregon, where hardwoods were scarce, the identical bird species nested mainly in Douglas-fir . Flammulated owls show similar differences over their range. Cavities in oaks (Quercus spp.) and trembling aspen were used where they were prevalent [189–191]; elsewhere in its range, the owl nests in ponderosa pine and Douglas-fir [192–194]. Similarly, where large, hollow hardwoods were not available, all cavities used by Vaux’s swift () were in large grand fir .
Flexibility is not universal and is constrained by foraging requirements. Nesting of some weak excavators (e.g., white-headed woodpecker; pygmy nuthatch, Sitta pygmaea) is largely restricted to conifers (Figure 5). Throughout its range, only 14 of 256 white-headed woodpecker nests have been reported from hardwoods. The restriction is related to a diet high in pine seeds [8, 93]. The pygmy nuthatch shows a similar strong and almost exclusive preference for long-needled pine forests . Restricted diets limit the acorn woodpecker and some sapsuckers (once trees leaf out) largely to hardwoods and other woodpeckers, such as black-backed and three-toed woodpeckers, to conifers (Figure 5). Weaker excavators, such as Lewis’s and white-headed woodpeckers, that nest and forage in coniferous trees, are potentially threatened by forest practices, because the conifers in managed forests may not become old and decayed enough to permit excavation. Both Lewis’s and white-headed woodpeckers are designated “at risk” in the PNW.
The general model predicts that selection of tree species should reflect the tree’s ability to compartmentalize decay, frequently leading to selection of hardwoods. Foraging needs modify selection, but selection for cavity sites still reflects trees’ species-specific decay patterns. In the PNW, that generally means hardwood species such as aspen, cottonwood, and birch (Betula) are preferred. Among hardwoods, primary excavators select decay-prone species over more decay-resistant species . Fungi commonly invade live aspen, softening the heartwood, while the sapwood remains unaffected. Douglas-fir may be more abundant in the area but often decays from the outside in; that is, decay softens the sapwood before it affects the heartwood [197, 198]. By the time the heartwood is sufficiently decayed to allow excavation, outer layers of wood are sloughing. Smaller, weaker excavators, such as chickadees and nuthatches, sometimes have no choice but to use decay-softened sapwood [72, 88, 199]. Where hardwood species are not common, birds excavate more dead conifers. Rot patterns change both regionally and with the mix of tree species, impeding ready transfer of findings among areas. In coastal forests, western hemlock often harbours heart rot [200, 201], while the more decay resistant western red cedar can contain butt rots . It appears to be the relative amounts of these fungi that shift nesting preferences between western hemlock and western red cedar. Similarly, where the root rot Armilleria sinapina creates butt rot in aspen, bat roosts may be as low as 10 cm above ground .
Preferences for hardwoods, coupled with regional differences in relative abundance of hardwoods, produce regional differences in use of hardwoods. These are illustrated for birds in Figure 5 and mammals in Table 1. The trend in declining proportions of hardwoods used by mammals as den or roost sites from boreal through inland to coastal forests reflects the relative abundance of hardwoods. Black bears are an exception; boreal hardwoods do not attain suitable diameters for bear dens. Without context, the data of Figure 5 do not make a strong case for selection of hardwood cavity sites by birds. Mean percentages of hardwoods as nest trees are 41.8% and 50.1% for strong and weak excavators, respectively. The context is that these species are using from 40 to 50% hardwoods as nest sites in forests where prevailing climate strongly favours conifers over hardwoods , and the large majority of the trees are conifer [163, 231]. Prediction 3, hardwood nest sites are sought preferentially, is affirmed.
6. Size of Tree
The general model states that decay is the major influence on nest site selection, and susceptibility to decay is a major factor in tree species selection (Section 5). Decay, when “walled off” by living wood, provides the initial cavity site and protection. Size confers additional protection (height above ground) and room. The broad prediction 4 of the model embraces more specific corollaries about selection for size.(i)Birds seek larger diameters (partly by seeking decay or older, larger trees; partly by seeking greater cavity height above ground).(ii)Selection for taller heights will be less evident than for larger diameter (due to stem taper and many nest trees and snags having broken tops).(iii)Larger bird species will tend to select larger, older nest trees (partly to seek more room, but primarily because they are better able to excavate so can pursue opportunities for greater protection in taller, larger trees). (iv)For the same bird species, diameters sought in nest trees will be larger in conifers than in hardwoods (largely because conifers usually require longer periods to attain rot, during which they attain greater size). (v)Relations between size of nest tree and size of species will be less apparent among secondary cavity users (because they “take what they can get”). (vi)No matter the animal’s size, most den trees of mammals will be large, especially in conifers (because they rely greatly on natural cavities; trees are older, thus larger; height above ground may be less important for mammals because they are better able to defend themselves than are birds).
I found 194 samples across 19 bird species that related nest tree diameter at breast height (dbh) to size of bird when nesting in either hardwood or coniferous trees. Larger diameter trees or snags are clearly preferred by cavity-nesting birds. In 30 of 31 comparisons, diameters of trees and snags used as nest trees were greater than the mean diameter of the available pool . The same bird species does tend to use larger diameters when nesting in conifers than in hardwoods (Figure 6). For example, mean dbh of nest trees of pileated woodpeckers nesting in inland conifers was 79.3 cm, but only 54.2 cm in inland hardwoods; comparable values from coastal sites were 77.9 and 44.5. cm. Across all strong excavators, mean nest tree diameters in conifers were 82.2 cm on the coast and 53.7 cm inland. Comparable values when nesting in hardwoods were 45.9 and 36.8 cm. Values for weak excavators nesting in conifers were 86.8 cm on the coast and 55. 3 cm inland; when nesting in hardwoods, values were 45.4 cm and 38.4 cm.
This pattern follows from patterns of rot. In the PNW, hardwoods generally rot at younger ages and smaller sizes than do conifers [197, 233]. Conifers, in particular, grow faster on the coast and attain larger sizes before rot has developed. The tendency for birds to select much larger conifers on the coast than in the interior affirms the pattern. Regional differences in size and growth rates are less for hardwoods, so interregional differences in size of hardwood trees used are less. Size of nest tree selected differs little between strong and weak excavators on the coast or inland.
Larger birds seeking larger trees is apparent only for inland forests (Figure 6). In coastal forests, there is little tendency for larger strong excavators to seek larger trees, because even the smallest of them sometimes excavate in very large trees. The latter observation suggests that coastal conifers attain large sizes before even small pockets of rot develop, again affirming the role of rot in nest tree selection. Both strong and weak excavators select larger conifers when nesting in conifers than when nesting in hardwoods. The tendency is stronger in small, weak excavators that require well-developed decay (Figure 6).
There is little relation between body size and secondary cavity nesters in conifers, but not in hardwoods (Figure 7). Even relatively small secondary nesters select large trees when nesting in conifers, there apparently is no need to do that in hardwoods. Nest tree diameters must be large enough to accommodate a cavity with room for an adult bird and nestlings, but sizes in conifers usually exceed that requirement. The selection of trees much larger than the size of cavity required reflects not only pursuit of height above ground, but age and the size at which heart rot develops. That occurs at younger ages and smaller sizes in hardwoods. Collated diameters of conifer nest trees of tree swallows ranged from 18 to 78 cm. Flammulated owls are only slightly larger than a sparrow, but nested in ponderosa pine averaging 57.7 cm dbh on southern aspects and 71.7 cm on northern aspects (data of ). The difference reflects greater rates of growth on north aspects, thus size at the age when rot appears. Pygmy nuthatches, flammulated owls, white-headed woodpeckers, and pileated woodpeckers all nest primarily in large ponderosa pine or Douglas-fir in at least some regions [57, 92, 192, 252], showing little relationship between size of bird and diameter of tree during nest site selection in conifers (Figures 6 and 7). I found few recorded diameters of conifer nest trees for larger species, such as waterfowl and the barred owl (Strix varia). Northern hawk owl (Surnia ulula), barred owl, bufflehead, hooded merganser (Lophodytes cucullatus), Barrow’s golden eye (Bucephala islandica), and wood duck (Bucephala islandica) nests were recorded primarily in hardwood species.
Milling efficiency constrains the sizes of trees grown in managed forests. Optimal milling efficiency varies with mill configuration, but for most sawmills in the PNW the preferred maximum log diameter is from <45 to 50 cm . That has been further lowered where salvage of trees killed by mountain pine beetles dominates the harvest. It has been suggested for inland forests that trees must be at least 23 cm dbh to provide a nesting site . Most reported values exceed 23 cm dbh (Figure 6), but a few smaller mean values have been reported for black-capped and mountain chickadees (Poecile gambeli), red-breasted nuthatches (Sitta canadensis), and downy woodpeckers (Picoides pubescens), especially when nesting in hardwoods (Figure 6). Mean diameters of conifer nest trees in inland forests consistently exceeded 50 cm dbh for several species, including red-breasted (Sphyrapicus ruber), yellow-bellied and Williamson’s sapsuckers (Sphyrapicus thyroideus), and pileated and white-headed woodpeckers. In all but one study, data for the northern flicker  and Lewis’s woodpecker  also exceeded 50 cm dbh.
Of these species, only the white-headed and Lewis’s woodpeckers are weak primary excavators, and several play keystone roles in particular regions. Three are candidates for designation or are designated “at risk” in portions of the PNW (Lewis’s, pileated and white-headed woodpeckers), and one has two subspecies designated (Williamson’s sapsucker). In coastal forests, some small birds (chestnut-backed chickadee, red-breasted nuthatch) consistently selected nest trees with a mean dbh >50 cm, indicating the strong role of age and decay in nest site selection by weak excavators (the chestnut-backed chickadee frequently nests in stumps when it cannot find large trees with rot). Habitat maintenance and milling efficiency are incompatible for some species.
Preferred roost trees also can exceed the maximum diameter for milling efficiency. Unlike nests, which often are excavated anew each year, roosts usually are sought in natural cavities. The average diameter of 123 pileated woodpecker roost trees was 71 cm . Most of these were grand fir extensively decayed by Indian paint fungus (Echinodotium tinctorium). Pygmy nuthatches use communal roosts housing up to 100 birds during winter . In Arizona, these roost sites averaged 73.2 cm dbh . Similarly, the hollow trees or snags used by Vaux’s swift must be large enough to allow the swift to fly up and down within the tree or snag. The mean dbh of 18 roost trees of Vaux’s swift was 77 cm .
Mammals do select large trees for cavity sites (Table 2). Emphasis here is on implications to forestry, so only studies for which ≥40% of roosting or denning sites were in tree cavities, or bark fissures are summarized in Table 2. In some areas, species such as the red squirrel (Tamiasciurus hudsonicus) and northern flying squirrel primarily use platforms or mistletoe brooms; some bat species (e.g., pallid bat, Antrozous pallidus, and fringed myotis, Myotis thysanodes) use a variety of substrates for roosting. For cavity or roost sites, even small bats use larger trees. Most studies of tree-using bats have found that species selected for larger diameters and greater heights than those of random trees . Data of Table 2 extend that finding. One result is that diameters and heights selected vary with region, generally increasing from subboreal and boreal forests through inland forest, attaining their greatest values in the coastal forests where trees are largest (Table 2). The pattern reflects the age, thus size, at which common tree species develop heart rot or deeply furrowed bark.
Differences in regional availability of large trees are most clear for the largest species in Table 2, black bear. Along the coast, from Vancouver Island south through Oregon, >95% of black bear dens were in large dead or dying trees and downed wood (Table 2 and ). In inland forests with trees of smaller stature, 69% of black bear dens were in wooden structures (Table 2). Mean dbh of hollow, black bear den trees is >100 cm in both coastal and inland forests. In boreal forests, only 12.3% of dens were in wooden structures. That value underestimates the significance of large trees to black bears. Of the 89 earthen den sites in boreal forest, 41% were under trees or stumps, 23% were under logs, and only 36% were directly into soil . Grizzly or brown bears also use hollowed trees, though infrequently, in British Columbia  and Europe . Most brown bear dens in Scandinavia were reported from ant hills and stumps .
Other than for the largest species, there is little relation between the size of tree selected and the size of the mammal. Bats use larger trees (Table 2), because they often use either natural hollows or cavities excavated by woodpeckers. The difference between hardwoods and conifers noted for birds applies (Figure 6). Diameters of bat roost trees on inland sites averaged 53.8 cm dbh () for conifers and 48.1 cm for hardwoods (; Table 2). Six mammal taxa in Table 2 that use trees or snags >50 cm are listed as “sensitive” or “at risk” in the PNW (Keen’s myotis, northern myotis, black bear (subspecies Kermodei), grizzly bear, fisher, and American marten).
Shape of the diameter distribution is more informative than the mean. Cumulative frequency distributions (CFDs) of snags tend to have long tails as small numbers of scattered large snags gradually accumulate (Figure 8). These tails raise mean values. That pattern is particularly evident in Figures 8(a) and 8(c). In coastal forests, active (used) snags were larger (height and diameter) than inactive or unused snags in old-growth areas (); snags in 70- to 100-year-old second growth were smaller than either active or inactive old-growth snags () . Only 14.7% of nest sites were located in snags <46 cm dbh (Figure 8(a)). Figure 8(b) illustrates the CFD of diameters of aspen nest trees over a range of about 15 to 55 cm dbh. Steep parts of the curves indicate the most sought diameters of nest trees in the region and are clearly evident in Figures 8(b) and 8(c). Most data on snag use are collected from unmanaged forests. Figure 8(d) compares nest trees selected by red-breasted nuthatches in managed and unmanaged forests of different tree species. The similarity of snag sizes selected suggests some governing relationship within the two disparate forest types.
Three points are apparent. First, birds show a central tendency (steep portion of the CFD) when selecting nest tree diameters, even across different forest types (see also ). Second, because of the long tails in many distributions, this tendency is better reflected by medians (which typically are smaller than means), but means are more often reported. Third, minimum nest tree diameter is a poor management target, because it ignores the central tendency. A more appropriate management target would be to provide trees larger than the median diameter at which heart rot commonly accrues in that forest type.
The fact that dbh of trees used for nesting may be much larger than the size of cavity needed by birds reflects the benefits of nesting high in trees to achieve greater protection from mammalian predators [113, 311]. In living trees, diameter and height usually are closely related. That is not true of snags because many have broken tops. Nonetheless, we expect cavity-nesting birds to seek out taller nest trees, both live and dead, from those available.
Taller snags generally are used disproportionately to their availability, but the mean height of nest trees exceeds that of the mean available only about half the time . That occurs because many snags with broken tops are used. For example, in interior Douglas-fir, 14 of 20 nests of red-breasted nuthatches were within 2 meters of the top of decay class 4 Douglas-fir snags with broken tops; 21 of 22 nests in Englemann spruce-Subalpine fir forest were in similar class 4 Subalpine fir snags, broken off by heart rot and wind .
On the coast, mean reported nest heights averaged 16.9 m, 14.10 m, and 10.1 m for strong primary excavators, weak primary excavators, and secondary users, respectively. Comparable values from inland sites were 9.4, 8.9, and 7.4 m (e.g., Figure 9). Small, weak excavators often use soft substrate, commonly found in short, broken-topped snags or stumps. That tendency is most apparent among chickadees and accounts for some of the short nest tree heights in Figure 9(b). Larger excavators, both weak and strong, also occasionally use older, broken-topped snags or stumps (e.g., northern flickers, downy, black-backed, three-toed, and white-headed woodpeckers in Figures 9(a) and 9(b)). Secondary users most often nest close to the ground (Figure 9(c)). Reported tree or snag heights are naturally taller than nest heights. On the coast they were 25.2 m, 16.5 m, and 18.1 m for strong primary excavators, weak primary excavators, and secondary users, respectively. Comparable values for inland sites were 19.5 m, 12.5 m, and 16.1 m. The relatively tall tree heights for some secondary users reflects the use of cavities in larger taller trees by larger species such as kestrels and owls.
Among strong excavators, larger bird species locate their nests higher above ground (), and there is a tendency for larger birds to locate nests in taller trees and snags (), but there is great variability due to broken tops (Figure 9(a)). Across all species, however, nest height shows little relation with size of the bird (). A general lack of relationship between bird size and nest height is expected, because stem taper means that larger cavities sought by larger birds cannot be as near the top of the tree as those of smaller birds. Management targets can be based on diameters alone.
7. Density and Distribution
There is little clarity on meaningful measures of density and distribution of trees for cavity nesters. Several issues, including scale, obscure appropriate metrics. For example, the area over which to measure density is ill defined and almost certainly a function of species-specific territory sizes, which themselves are a function of foraging opportunities; many cavity-using species use trees or snags for foraging as well as nesting (forage and nest trees have different attributes, but both are necessary); primary and secondary users of cavities usually forage differently; there are tradeoffs in the relative use of hardwoods and conifers, but researchers sometimes report only conifers; and numbers of snags in the immediate area of the nest tree likely overestimate requirements, because snags tend to occur in clumped patterns that do not extrapolate over large areas.
Predictions of expected responses are thus limited.(i)Responses to density will be inconsistent and better exposed by extremes in simpler managed stands than in highly variable natural or near-natural stands.(ii)Cavity nester density will be asymptotic against nest tree density and limited by factors other than snag density as snag density increases. (iii)There will be little response to snag density in hardwood stands because a high proportion of living hardwoods contains heart rot.(iv)Clumped distributions of nest trees (including snags) will better sustain primary excavators, because they are more likely to include foraging requirements.(v)Many secondary users will favour dispersed retention because they forage in more open areas.
Density of cavity-nesting birds is inconsistently related to measures of snag density. Generally, studies surveying natural stands with many snags obtain weak relations (e.g., 27, 61, and 316), while those including managed stands, in which some or all snags were removed, find stronger relations (e.g., [54, 70, 324]). In managed forests where snag density was high, there was no relation between cavity nester and snag density . Including simpler managed stands in analysis is more revealing of snag effects than using only natural stands. Within stands ranging from unmanaged to scattered seed trees, density of snags 25–50 cm dbh was a strong predictor of red-breasted nuthatch and chickadee densities; and 0.83, respectively (mountain and chestnut-backed chickadees combined) . Such relationships may indicate consistent nest site limitation or better foraging opportunities in stands with more snags (e.g., older stands with more insects versus thrifty managed stands).
Figure 10 illustrates relationships for stands dominated by conifers. Most variation in cavity nester response to snag density is due to sizes of the snags, so estimates of density were limited to larger snags that the birds use preferentially: ≥50 cm dbh for coastal forests and ≥30 cm for inland forests. Presence of an asymptote in density was evaluated by fitting a Michaelis-Menten relationship to data illustrated in Figure 10(a). That invokes the assumption that the rate of response (cavity nester density) is a function of the concentration of substrate (snags/ha). Response of the seven studies combined is asymptotic, with little additional increase in density of cavity nesters beyond about 3 large snags per ha (Figure 10(a)). That shape is expected among species limited by resources such as food, space, or nest sites. Birds present at 0.0 snags per ha indicate that some species can nest in snags <30 cm dbh (some of them hardwoods, unreported in data on conifers).
The response shown in Figure 10(a) was consistently expressed: each individual study shows an initial increase in cavity-nester density with increasing snag density. That merely affirms large snags become limiting at densities of ≤1 per ha. The fact that the relation fits a Michaelis-Menten relationship implies that the response is consistent with limitation by substrate abundance. Small differences would shift the estimated half-saturation value between 2 and 3 large snags per ha. Relationships for some smaller individual bird species were more linear (Figures 10(b) and 10(c)). Territorial behaviour is only weakly expressed in the chestnut-backed chickadee but is evident in mountain chickadees. It is possible that competition for cavity sites is more strongly expressed within the entire cavity-nesting fauna, than within individual small species. However, data of Figure 10(c) also suggest that other variables may influence the response; at the same snag densities, data of  are consistently higher than those of . During a mountain pine beetle epidemic, densities of both red-breasted nuthatches and mountain chickadees increased dramatically and linearly as beetles killed more conifers and provided more food. Once most of the conifers were killed, numbers of conifer snags continued to increase, but the numbers of beetles diminished and so did the nuthatches and chickadees . Although the density of conifer snags increased greatly, the birds continued to nest primarily in aspen; food was a more influential variable than snag density.
Relations of Figure 10 are helpful to the extent that they reveal that low snag densities are limiting and suggest that about 2.5 large snags per ha support 50% of natural densities of cavity nesters. Current snag-retention guidelines for most North American forest types fall between 1 and 10 large snags/ha, converging around 6 to 7 trees/ha . These guidelines derive primarily from work in the Blue Mountains of Oregon and are expected to sustain the full complement of cavity nesters (: Appendix 22], ). Because of the great variation in the data, they differ little from the relationship of Figure 10.
There is a problem however. Although small and large snags are both important to cavity nesters, nesting densities usually correlate better with large snag densities than with densities of all snags. For example, in the coastal rain forest of British Columbia densities of all snags did not differ between old-growth sites and 70-year-old second growth (34.8 and 33.9 snags/ha, respectively . Because many snags were larger in old growth, the proportion of snags used was about 12 times greater (16.0%, compared to 1.3% in second growth). Median diameter was about 18 cm dbh over all second-growth snags and 62 cm dbh for nest trees in old growth. Relations derived from the Blue Mountains and in Figure 10 address nesting, so focus on larger snags. Woodpeckers, however, tend to forage on several smaller snags rather than the single larger snag in which they nest. Smaller snags that represent foraging sites, but are inadequate for nesting, were present in all studies included in Figure 10 and undoubtedly influence the apparent response to snag density. Their abundance often is not reported. The number of small snags required, and the preferable distribution of snags remain obscure. The former is troubling because small snags are necessary foraging sites for many species. The latter can be evaluated by more integrative measures of retention (see below).
Data are few, but snag density appears less important in hardwood stands. In southern hardwood stands of Florida, cavity nester density increased with density of hardwood snags, though not as dramatically as in pine forests ; there was no increase in cavity nesters with a doubling in hardwood snags in mesophyll forest in Kentucky . In mixed forests of hardwood and conifer, five of six cavity-nesting species increased dramatically during a mountain pine beetle epidemic and greater foraging opportunities . Although the number of conifer snags increased dramatically, the birds continued to locate 95% of their nests in living or recently dead aspen. Densities of the five cavity nesters responding increased 5- to 6-fold with little change in density of hardwood snags. Black-capped chickadees, which did not change in density, forage primarily on insects found on living trees and showed no response to increasing numbers of beetles or conifer snag density until most trees were dead (when chickadees declined in abundance).
Distribution has both spatial and temporal dimensions. From the broadest to finest scale, questions we ask about distribution of a resource are ordered. How do we sustain a continuous supply? Should actions be focused at the landscape level or within stands? How should within-stand efforts be distributed? How much is necessary? What kinds (e.g., sizes)?
Kinds and amounts (how much) were addressed above. Here we address the three broader questions noted. The broadest scale of distribution relates to planning for the sustained contribution of dying and dead wood. We know that if a 200-year-old tree is required 200 years from now, it must be planted now and subsequently reserved from harvest. Models that project the creation and duration of snags reveal projected shortfalls large enough to significantly impact cavity nesters [2, 3, 335]. For example, projections for managed Douglas-fir forest suggest 0.1 large snags per ha per decade , well below required levels of Figure 10. These models typically address the temporal scale and indicate that in managed stands snags are not being created or replenished fast enough to ensure a continuous supply. The solution appears simple—leave more trees to become snags. Solution becomes more complicated when we include the spatial scale. There often are landscape-level reserves or set asides (e.g., steep slopes, riparian areas, and reserves for a particular species) that contribute snags now and in the future. Trees and snags also may be retained within stands.
The challenge is then to determine what should be retained within stands, given landscape-level reserves and the natural rates of snag creation or fall, and how should within-stand retention be distributed? The kinds of bookkeeping and projection required are relatively simple, but we confront several challenges: (1) we usually do not have good estimates of rates of key processes (e.g., snag creation or tree mortality rates, snag fall down rates, and progression of decay or period when the snag is useful), (2) dead wood is a more transient habitat that many others and organisms are adapted to particular transient stages of decay (Figure 2 and ), (3) the diverse forms of tree mortality ensure stochasticity and a great deal of variation when amounts in natural stands are sampled [307, 336–338], (4) species using dead wood respond variably and may seek different sites for nesting and foraging, and, perhaps most challenging, (5) scale, which makes amount and distribution nearly inseparable. The same amount can be patchily or uniformly distributed. Patchily distributed amounts may have too little dead wood at local scales and what is local varies among species.
It is clear that many managed forests produce too few large nest trees, but once we choose to retain trees, the questions of where that retention should occur and what form it should take remain. Tolerance levels can help address the natural variation in measures of amount . The issue of distribution remains. Retaining trees as landscape reserves or within harvest blocks have different costs. We need to know whether they also have different consequences. The shape of response curves relating cavity-nester abundance to proportion of forest retained permits evaluation of tradeoffs between stand- and landscape-level retention. The response of cavity nester abundance could be directly proportional to the proportion of forest retained, above or below proportional, or reveal potential thresholds (Figure 11).
The nature of the response curve has implications to the distribution of retention:(i)Directly proportional to the amount of cover retained implies that within-stand and landscape-level retention contribute equally. The total abundance of the species in a landscape depends only on the total retention level, whether this is retained within stands or in landscape reserves or in a combination of the two. (ii)Above proportional to the amount of cover retained indicates that within-stand retention during harvest is creating greater opportunities for the species within each stand. For example, in Figure 11 the >proportional response indicates that retaining 20% of trees in a cut block maintains a species at 40% of its abundance in uncut forest. That supports using retention in cut blocks at the stand level. For a given total amount of retention, stand-level retention maintains a greater abundance of the species across the landscape than would alternative strategies, such as retaining the same total amount in large reserves and clearcutting the harvested stands (all else being equal). (iii)Total abundance less than proportional indicates that landscape-level retention is favoured because retention is less useful when “diluted” within individual cut blocks. For example, in Figure 11 the <proportional response indicates that 20% retention maintains the species at only 5% of its abundance in uncut forest. To sustain species showing such a response, it is better to allocate a given retention level to larger reserves, rather than stand-level retention (again, all else being equal).
The most helpful response to management would be a marked threshold response, that is, a sudden decline in a species’ abundance when amount of a habitat variable drops below a particular value [341–343]. If well-defined thresholds were evident, they would help guide choices of target levels for retention . However, the many sources of variability in natural systems means thresholds are more likely to occur as softer, sigmoidal-type relationships (Figure 11, [341, 343]). Whereas a hard threshold reveals a clear target value, a sigmoidal response or “softer” threshold suggests a mixed strategy of stand- and landscape-level retention, or of higher and lower retention levels in harvested areas. The practical point is whether the relationship has a point of inflection, where abundance changes from more to less rapidly increasing, as retention levels increase. For example, with 20% overall allocation to retention, the sigmoidal relationship in Figure 11 implies the best strategy would be to retain 35% in about half the stands with minimal retention in the rest.
In fact, most conceivable shapes of response appear in nature . Few generalities are apparent among the 15 cavity-nesting species for which meta-analysis was possible: only 1 species (brown creeper in hardwoods) showed a response less than proportional to retention; 5 species showed neutral responses (these sometimes limited to particular seasons or regions); the majority profits from some within-stand retention. For some species, retention had no apparent effect until it reached levels from 15 to 20% of the original volume; others profited from some opening of the canopy. The combined response of 15 species plus 95% confidence intervals is illustrated in Figure 12.
In Figure 12, the “community” is a composite of studies of 15 species , each standardized as 100% abundance in uncut forest. It shows that even small amounts of harvest eliminate some species, but over a wide range of retention most species are present, with a few species occurring much more commonly at high retention levels (openings <20% canopy cover). These few species—black-capped chickadee, mountain chickadee, downy woodpecker and red-breasted sapsucker—produce the wide confidence limits in abundance at higher retention levels. Clearly, some cavity nesters profit from opening of the canopy (abundance greater than that in uncut forest), and, other than for a few species, little is gained in abundance beyond 30% retention (Figure 12(a)). Collating data from several sources ensures wide scatter at the lower end, so confidence limits do not converge to near zero at zero retention. For example, northern flicker forages on the ground and is the only cavity nester for which confidence intervals do not contract towards zero abundance at zero retention at the stand level.
Even though desired amounts of retention vary across species, it is clear that all cavity nesters profit from some degree of retention. Retention silvicultural systems (e.g., green-tree retention and variable retention) contribute permanent retention—the trees will never be harvested. Other partial harvesting systems, such as regular or irregular shelterwoods, selection cuts, strip cuts, and group removals, often leave high levels of retention, but the trees are not intended to be retained permanently. Whatever the approach, trees retained within a treatment unit can be distributed in groups or as scattered individuals (dispersed retention) or as a mix of both.
Review of studies that permitted comparisons of forms of retention revealed three major patterns . First, although dispersed retention of snags (high levels of removal) increased abundance and richness of secondary cavity nesters beyond that found in mature and old-growth forests, abundance of primary excavators was reduced (see also ). This response accounts for the increase in similarity of combined species at the highest levels of retention in Figure 12(b). Group retention better sustains primary excavators. In both logged and managed stands, nests of primary excavators often are concentrated in dense patches of snags or broken-topped trees for both strong and weak excavators [70, 72, 74, 244]. Woodpeckers also used snags in groups, even in an area where snags were uniformly distributed by salvage logging . Likewise, there was no difference in nesting levels between clustered and scattered snags that had been created by topping, suggesting that some natural phenomenon was acting . Trees, snags, and down wood typically occur in clumps . It is unclear whether primary excavators seek dense patches of nest trees for some associated value or if it merely reflects the patchy way in which trees are killed by insects or disease.
Second, secondary cavity nesters were more abundant in areas experiencing some timber removal than in mature or old-growth forests. That increase likely reflects the fact that many secondary nesters forage more effectively in openings and accounts for the increase in abundance at midlevels of retention in the combined response of Figure 12(a). Third, in most instances the abundance of primary cavity nesters was little affected by partial harvesting, and in some instances increased in abundance. That may reflect the fact that several primary excavators also favor small openings and edges (e.g., hairy woodpeckers , northern flicker , three-toed woodpecker , and perhaps downy woodpecker ). In central British Columbia, preferred nest sites for three-toed woodpeckers were within 20 meters of an edge .
In summary, it is apparent that snags can be limiting and that from 2 to 3 appropriately sized snags per ha appear to sustain about 50% of the natural cavity-nesting community locally (Figure 10). Individual species, however, show a variety of responses (Figure 12), with broad generalities apparent only between primary and secondary nesters. It is much less clear how much of the forest should have what snag densities. Sustained provision of habitat will differ among forest types, and the rates at which snags are produced, decay and fall. Estimating total snags required must recognize that smaller snags are used as foraging sites, and foraging sites are sometimes more limiting than cavity sites [350, 351]. More dead or dying wood than required for nesting is needed to sustain all cavity-nesting species. Moreover, through provision of perching, foraging, and hawking sites, snags of all sizes tend to increase richness and abundance of birds other than cavity nesters [352, 353].
8. Management Implications
In forests of the PNW, most studies find significant correlations between snag density and the abundance of primary excavators (Figure 10 and review of ). Similar relations are evident for secondary cavity users . Because we prefer that trees in managed forests do not die or become rotten, management is designed to reduce snag densities below natural levels [2, 3, 335]. One result is that a large portion of cavity-using species is listed “at risk,” or “potentially at risk,” globally  and in the PNW . Over the past two decades, greater effort has been made to provide snags in managed forests (e.g., [355, 356]). Data collated here can help to guide those efforts. Comprehensive regional repositories for dead wood data and species using dead wood are available, such as DecAid for Washington and Oregon .
Major challenges to extracting management guidelines from available data are the fact that amounts and distribution of trees and snags are naturally interrelated, and that concern for old growth, particularly in the PNW, has focused research on older forests. The latter condition means that many studies report use based on a wide range of tree sizes; the results are “natural” but potentially inflate diameters required by the species. That is evident in regional differences in nest tree diameters (Table 1). The patterns of cavity use by birds and mammals extracted by this review cannot evade those challenges but still yields general guidelines to assist efforts at sustaining cavity-using species.
Implications following were derived from review of more than 350 studies of 67 cavity-using species occurring in the PNW. The regional focus was intended to restrict the number of species to a naturally occurring assemblage; data on use by a given species were incorporated no matter where the study occurred, including Eurasia. Varied locations act as independent samples of apparent use and preference. Even within the PNW, there are clear regional differences in patterns of nest or roost tree selection (Tables 1 and 2), much of it due to the relative abundance of hardwood and conifer species that are used differently (Figures 2, 5, 6, 7, and 9). Documenting biological reasons for regional differences within a general model aids extrapolation beyond the PNW.
8.1. Ensure Sustained Provision of Dying and Dead Trees
Many species depend on dying and dead trees and eventually logs (Figures 3 and 4). Snags come from living trees. If a species will need a 150-year-old tree that is required 150 years from now, that tree must begin growth now. Planning to sustain cavity sites must consider not just what is there now, but how that will be sustained. A practical way of providing sustained diversity of structure, including dead trees, is patchwise or group retention (; e.g., Figure 12).
8.2. Retain Trees and Snags of Both Hardwoods and Favoured Conifer Species
Because hardwoods generally are preferred cavity sites (Figure 5) and provide favourable substrate at a younger age than do conifers, it is tempting to rely on hardwoods to meet cavity users’ needs. Although they are favored nesting sites, we cannot rely solely on the more decay-prone hardwoods. Hardwoods are uncommon in many forests of the PNW and the varied needs of forest organisms include well-decayed snags, large hollow snags, and snags with loose slabs of bark. Hardwood species will not accommodate all these needs, nor will any one species of conifer. Conifers are longer-lived and provide a longer-lasting source of cavities than do hardwoods [358, 359], so are more likely to sustain snags late into rotations. Moreover, conifer snags are required by species foraging on bark beetles or wood-boring beetles and conifer logs last longer than do hardwood logs. Favoured conifers include western larch, Douglas-fir, and ponderosa pine. Monocultures of less preferred species, such as lodgepole pine, should be avoided.
8.3. Sustain a Range of Decay Classes of Potential Nest Trees
A range of decay classes in standing trees is necessary to sustain biodiversity (fungi, arthropods, birds, and large mammals; e.g., Figure 2). The same is true for species using down wood [307, 360]. Regular inputs from harvest residues are important to maintain the total amount of down wood, but that input tends to be concentrated in a narrow range of decay classes. Retention helps sustain recruitment and ensure continual provision of different decay stages in standing and down wood. Given the patterns of succession among species on and within logs, gaps in recruitment should not greatly exceed 25 years . Organisms in standing trees also use a range of decay classes (Figure 2) and similar recruitment intervals appear appropriate for sustaining standing decay. Some recruitment happens naturally within landscape-level reserves or set-asides (steep slopes, riparian areas, and reserves for particular species) and where retention is practiced. Sustained recruitment over large areas involves planning to avoid large gaps in size and decay stages as well as practices such as retention. Staggering harvest stages among adjacent stands helps sustain a variety of decay classes.
8.4. Retain a Range of Size and Age Classes
Large trees and snags provide nesting or denning sites longer than do small snags [42, 361]. However, smaller snags provide foraging sites, and many more foraging sites are needed than nesting sites. Although larger diameters usually are selected by vertebrates as cavity sites, smaller trees and logs are used. Where safety considerations encourage falling older snags during harvest, managers should ensure that snags can develop through the rotation. The desirability of a range of decay classes is well documented for bryophytes, insects, terrestrial breeding salamanders, and birds [1, 307]. Well-decayed snags present greater safety risks and are more easily retained in patches. Unless reserve patches are very large, recruitment of well-decayed snags must occur outside of reserve patches. Snags may never become well decayed if operational guidelines require snag falling. Either no-work zones are required during subsequent entries, or silvicultural systems that do not require frequent entries should be employed in at least some areas. Retaining declining live trees, or recently-dead snags, ensures provision of later stages of decay.
8.5. Ensure That Some Large Trees or Snags Are Retained
Some large trees should be retained during both thinning and harvesting operations. Although individual species use a wide range of tree or snag sizes, they tend to select larger ones when available (Figures 6 and 7). In the PNW, studies of vertebrate-forest relations have been concentrated where trees are larger and more valuable, so existing data overestimate requirements where trees are smaller. Although data reveal many instances, particularly in coastal forests, where birds have selected larger trees, most species can be accommodated in managed forests by conifer cavity trees 50 cm dbh; hardwood trees can be smaller, and a dbh >30 cm accommodates most bird species in less productive, inland forest types . However, those values appear too small for some bird species, including four designated “at risk” (Figure 6). Likewise, some mammals, including 6 taxa “at risk,” select trees or snags >50 centimeters in diameter or use down wood from 50 to 150 cm in diameter [125, 134, 362]. A major implication is that stands managed intensively for fibre production often will not retain all cavity users. That implies some portion of the landscape should be left unmanaged. There is no unequivocal estimate of how much. While single target diameters are the simpler management target, tolerance intervals accommodate natural variation and the potential bias of large amounts of data collected from old growth forests . Results of Figure 9 affirm that diameter is a sufficient guideline in assessing trees or snag suitability; height merits less attention.
8.6. Provide Both Aggregated and Dispersed Retention of Living and Dead Trees
Figure 10(a) suggests that relatively little is gained by providing more than 3 large snags per ha, but individual species may respond positively to greater snag densities (Figures 10(b) and 10(c). All forms of response in Figure 11 were observed, indicating that there is no single best approach. When targets are the cavity-nesting community, rather than a single species, about 30% retention maintains about 70% of community members (Figure 12(b)). The importance of variety in the distribution of retention applies equally to species reliant on downed wood . If management for cavity users is to be effective at larger scales, approaches must be developed over a scale appropriate to the range of species affected and recruitment rates of snags, rather than individual land ownership.
8.7. Meet Dead Wood Requirements for Larger Species in Areas Where the Emphasis Is Not on Intensive Fiber Production
There are economic and ecological advantages to zoning the intensity of fiber production [363, 364]. In some forest types, larger mammals prefer significant amounts (100 to 200 m3/ha or more) and large sizes (>50 centimeter diameter) of down wood (Table 2; ). Needs of such species are best provided in areas where late-successional attributes are being maintained. Provision of some large trees and pieces of dead wood in forests where the dominant goal is fiber production may facilitate dispersal among areas of more favorable habitat. Given the mobility of birds and patterns of larger mammals’ use of space, large snags and subsequent pieces of down wood for such species can be well distributed across large areas.
8.8. Do Not Do the Same Thing Everywhere
Retention of trees in patches reduces safety risks of snag retention and windthrow [366, 367] and facilitates retention of a range of size and decay classes. It also concentrates recruitment of down wood. Some organisms favour piles of down wood, while others favour scattered pieces (review in ). Dispersed retention of individual snags, or declining live trees intended to become snags, may be particularly advantageous for perching birds and for territorial secondary users, such as raptors and some small birds, but can impact shrub nesters negatively by encouraging aerial predators . Any single approach will disadvantage some group of species, so a range of practices is preferable when a range of species is to be sustained in an area.
8.9. Limit Salvage Logging after Tree Mortality
Concern for biodiversity led to restrictions on green-tree harvest, particularly in the PNW. That in turn encouraged more complete harvest of dying and dead trees in areas subject to insect attack or fire kill. We learned two things. First, if all vertebrates are to be sustained in forests experiencing mortality to insects or fire, remaining standing dead trees should not all be logged. Review of more than 200 studies addressing forest harvest in beetle-killed forests  reported negative effects on bird, mammal, and fish species. Similar findings emerged from review of logging postfire areas [331, 370]. A second major point emerging from these reviews is that postdisturbance variation is so large that small-scale studies are unlikely to provide general principles for mitigating damage, beyond “do not take all dead trees and do not do the same thing everywhere.”
This work was partially supported by the Forest Sciences Program of British Columbia. Several individuals provided published data in raw form, so that the data could be included in figures, specifically Figures 2, 3, 4, 5, 6, 7, and 9. The author is grateful to B. Booth, J. Deal, D. Huggard, W. Klenner, M. Machmer, K. Martin, K. Squires, and C. Steeger for providing data. D. J. Huggard prepared Figure 12 from data he acquired during his review . I. Houde, L. Kremsater, and K. Squires helped collate the scattered data. A. Farr and three anonymous reviewers improved the paper.
- B. G. Jonsson, N. Kruys, and T. Ranius, “Ecology of species living on dead wood—lessons for dead wood management,” Silva Fennica, vol. 39, no. 2, pp. 289–309, 2005.
- C. D. Oliver, C. Harrington, M. Bickford, et al., “Maintaining and creating old growth structural features in previously disturbed stands typical of the Eastern Washington Cascades,” Journal of Sustainable Forestry, vol. 2, no. 3, pp. 353–387, 1994.
- G. F. Wilhere, “Simulations of snag dynamics in an industrial Douglas-fir forest,” Forest Ecology and Management, vol. 174, no. 1–3, pp. 521–539, 2003.
- A. Berg, B. Ehnstrom, L. Gustafsson, T. Hallingback, M. Jonsell, and J. Weslien, “Threatened plant, animal, and fungus species in Swedish forests: distribution and habitat associations,” Conservation Biology, vol. 8, no. 3, pp. 718–731, 1994.
- J. Pojar, K. Klinka, and D. V. Meidinger, “Biogeoclimatic ecosystem classification in British Columbia,” Forest Ecology and Management, vol. 22, no. 1-2, pp. 119–154, 1987.
- L. W. Spring, “Climbing and pecking adaptations in some North American woodpeckers,” The Condor, vol. 67, no. 6, pp. 457–488, 1965.
- K. E. H. Aitken and K. Martin, “The importance of excavators in hole-nesting communities: availability and use of natural tree holes in old mixed forests of western Canada,” Journal of Ornithology, vol. 148, supplement 2, pp. S425–S434, 2007.
- E. L. Bull, S. R. Peterson, and J. W. Thomas, “Resource partitioning among woodpeckers in northeastern Oregon,” USDA Forest Service—Research Note, vol. 444, pp. 1–18, 1986.
- K. L. Cockle, K. Martin, and T. Wesołowski, “Woodpeckers, decay, and the future of cavity-nesting vertebrate communities worldwide,” Frontiers in Ecology and the Environment, vol. 9, no. 7, pp. 377–382, 2011.
- G. C. Daily, P. R. Ehrlich, and N. M. Haddad, “Double keystone bird in a keystone species complex,” Proceedings of the National Academy of Sciences of the United States of America, vol. 90, no. 2, pp. 592–594, 1993.
- D. S. Dobkin, A. C. Rich, J. A. Pretare, and W. H. Pyle, “Nest-site relationships among cavity-nesting birds of riparian and snowpocket aspen woodlands in the northwestern Great Basin,” Condor, vol. 97, no. 3, pp. 694–704, 1995.
- W. L. Foster and T. Tate Jr., “The activities and coactions of animals at sapsucker trees,” Living Bird, vol. 5, pp. 87–113, 1966.
- R. S. Miller and R. W. Nero, “Hummingbird- sapsucker associations in northern climates,” Canadian Journal of Zoology, vol. 61, no. 7, pp. 1540–1546, 1983.
- G. D. Sutherland, C. L. Gass, P. A. Thompson, and K. P. Lertzman, “Feeding territoriality in migrant rufous hummingbirds: defense of yellow-bellied sapsucker (Sphyrapicus varius) feeding sites (Selasphorus rufus),” Canadian Journal of Zoology, vol. 60, no. 9, pp. 2046–2050, 1982.
- R. T. Holmes, “Ecological and evolutionary impacts of bird predation on forest insects: an overview,” Studies in Avian Biology, no. 13, pp. 6–13, 1990.
- J. W. Thomas, R. G. Anderson, C. Maser, et al., “Snags,” in Wildlife Habitats in Managed Forests. The Blue Mountains of Oregon and Washington, J. W. Thomas, Ed., pp. 60–77, USDA Forest Service Agricultural Handbook, 1979.
- F. L. Bunnell and L. A. Dupuis, “Riparian habitats in British Columbia: their nature and role,” in Riparian Habitat Management and Research, K. H. Morgan and M. A. Lashmar, Eds., pp. 7–21, Special Publication of the Fraser River Action Plan, Canadian Wildlife Service, Delta, Canada, 1995.
- R. B. Bury, “Differences in amphibian populations in logged and old growth redwood forest,” Northwest Science, vol. 57, no. 3, pp. 167–178, 1983.
- J. J. Stelmock and A. S. Harestad, “Food habits and life history of the clouded salamander (Aneides ferreus) on northern Vancouver Island,” Syesis, vol. 12, pp. 71–75, 1979.
- P. H. Baldwin and N. K. Zaczkowski, “Breeding biology of the Vaux Swift,” The Condor, vol. 65, no. 5, pp. 400–406, 1963.
- C. M. Davis, “A nesting study of the Brown Creeper,” Living Bird, vol. 17, pp. 237–263, 1978.
- P. R. Ehrlich, D. S. Dobkin, and D. Wheye, The Birder’s Handbook: A Field Guide to the Natural History of North American Birds, Simon and Schuster, New York, NY, USA, 1988.
- L. de K. Lawrence, “A comparative life-history study of four species of woodpeckers,” Ornithological Monographs, no. 5, pp. 1–156, 1967.
- E. L. Walters, E. H. Miller, and P. E. Lowther, “Yellow-bellied Sapsucker (Sphyrapicus varius),” in The Birds of North America Online, A. Poole, Ed., Cornell Lab of Ornithology, Ithaca, NY, USA, 2002.
- M. C. Kalcounis and M. R. Brigham, “Secondary use of aspen cavities by tree-roosting big brown bats,” Journal of Wildlife Management, vol. 62, no. 2, pp. 603–611, 1998.
- S. E. Lewis, “Roost fidelity of bats: a review,” Journal of Mammalogy, vol. 76, no. 2, pp. 481–496, 1995.
- F. L. Bunnell and A. C. Allaye-Chan, “Potential of ungulate winter-range reserves as habitat for cavity-nesting birds,” in Proceedings of the Symposium on Fish and Wildlife Relationships in Old-Growth Forests, W. R. Meehan, T. R. Merrell, and T. A. Hanley, Eds., pp. 357–365, American Institute of Fishery Research Biologists, 1984.
- R. N. Conner, O. K. Miller, and C. S. Adkisson, “Woodpecker dependence on trees infected by fungal heart rots,” The Wilson Bulletin, vol. 88, no. 4, pp. 575–581, 1976.
- A. S. Harestad and D. G. Keisker, “Nest tree use by primary cavity-nesting birds in south central British Columbia,” Canadian Journal of Zoology, vol. 67, no. 4, pp. 1067–1073, 1989.
- L. Kilham, “Reproductive behavior of yellow-bellied sapsuckers. I. Preference for nesting in Fomes infested aspens and nest hole interrelations with flying squirrels, raccoons, and other animals,” The Wilson Bulletin, vol. 83, no. 2, pp. 159–171, 1971.
- A. L. Shigo and L. Kilham, “Sapsuckers and Fomes ignarius var. populinus,” USDA Forest Service Research Note, vol. NE-RN-84, pp. 1–2, 1968.
- M. O. G. Eriksson, “Clutch sizes and incubation efficiency in relation to nest box size among goldeneyes Bucephala clangula,” Ibis, vol. 121, no. 1, pp. 107–109, 1979.
- M. R. Evans, D. B. Lank, W. S. Boyd, and F. Cooke, “A comparison of the characteristics and fate of Barrow's Goldeneye and Bufflehead nests in nest boxes and natural cavities,” Condor, vol. 104, no. 3, pp. 610–619, 2002.
- L. Gustaffson and S. G. Nilsson, “Clutch size and breeding success of pied and collared flycatchers Ficedula spp. in nest boxes of different sizes,” Ibis, vol. 127, no. 3, pp. 380–385, 1985.
- J. H. Van Balen, “The relationship between nest-box size, occupation and breeding parameters of the great tit (Parus major) and some other hole- nesting species.,” Ardea, vol. 72, no. 2, pp. 163–175, 1984.
- K. E. H. Aitken, K. L. Wiebe, and K. Martin, “Nest-site reuse patterns for a cavity-nesting bird community in interior British Columbia,” Auk, vol. 119, no. 2, pp. 391–402, 2002.
- C. E. Bock, A. Cruz, M. C. Grant, C. S. Aid, and T. R. Strong, “Field experimental evidence for diffuse competition among southwestern riparian birds,” American Naturalist, vol. 140, no. 5, pp. 815–828, 1992.
- L. von Haartman, “Adaptation in hole-nesting birds,” Evolution, vol. 11, no. 3, pp. 339–347, 1957.
- E. L. Bull, “Longevity of snags and their use by woodpeckers,” in Proceedings of the Snag Habitat Management Symposium, J. W. Davis, G. A. Goodwin, and R. A. Ochenfels, Eds., pp. 264–67, USDA Forest Service General Technical Report, RM-GTR-99, 1983.
- R. Everett, J. Lehmkuhl, R. Schellhaas et al., “Snag dynamics in a chronosequence of 26 wildfires on the east slope of the cascade range in Washington State, USA,” International Journal of Wildland Fire, vol. 9, no. 4, pp. 223–234, 2000.
- S. M. Garber, J. P. Brown, D. S. Wilson, D. A. Maguire, and L. S. Heath, “Snag longevity under alternative silvicultural regimes in mixed-species forests of central Maine,” Canadian Journal of Forest Research, vol. 35, no. 4, pp. 787–796, 2005.
- M. L. Morrison and M. G. Raphael, “Modeling the dynamics of snags,” Ecological Applications, vol. 3, no. 2, pp. 322–330, 1993.
- R. E. Russell, V. A. Saab, J. G. Dudley, and J. J. Rotella, “Snag longevity in relation to wildfire and postfire salvage logging,” Forest Ecology and Management, vol. 232, no. 1–3, pp. 179–187, 2006.
- J. K. Agee, Fire Ecology of Pacific Northwest Forests, Island Press, Washington, DC, USA, 2003.
- F. L. Bunnell, “Forest-dwelling vertebrate faunas and natural fire regimes in British Columbia: patterns and implications for conservation,” Conservation Biology, vol. 9, no. 3, pp. 636–644, 1995.
- F. L. Bunnell, I. Houde, B. Johnston, et al., “How dead trees sustain live organisms in western forests,” in Proceedings of the Symposium on the Ecology and Management of Dead Wood in Western Forests, W. F. Laudenslayer Jr., P. J. Shea, B. E. Valentine, C. P. Weatherspoon, and T. E. Lisle, Eds., pp. 291–318, USDA Forest Service, General Technical Report PSW-GTR-181, 2002.
- W. G. Dahms, “How long do ponderosa pine snags stand?” in Research Note, pp. 1–4, USDA Forest Service, Pacific Northwest Forest and Range Experiment Station, Portland, Ore, USA, 1949.
- F. P. Keen, “How soon do yellow pine snags fall?” Journal of Forestry, vol. 27, no. 6, pp. 735–737, 1929.
- J. M. Schmid, S. A. Mata, and W. F. McCambridge, “Natural falling of beetle-killed ponderosa pine,” pp. 1–3, USDA Forest Service Research Note RM-454, 1985.
- S. Zack, T. L. George, and W. F. Laudenslayer Jr., “Are there snags in the system? Comparing cavity use among nesting birds in “snag-rich” and “snag-poor” eastside pine forests,” in Proceedings of the Symposium on the Ecology and Management of Dead Wood in Western Forests USDA Forest Service, General Technical Report PSW- GTR-181, W. F. Laudenslayer Jr., P. J. Shea, B. E. Valentine, C. P. Weatherspoon, and T. E. Lisle, Eds., pp. 179–191, 2002.
- K. Martin, A. Norris, and M. Drever, “Effects of bark beetle outbreaks on avian biodiversity in the British Columbia interior: implications for critical habitat management,” BC Journal of Ecosystems and Management, vol. 7, no. 3, pp. 10–24, 2006.
- J. Schepps, S. Lohr, and T. E. Martin, “Does tree hardness influence nest-tree selection by primary cavity nesters?” Auk, vol. 116, no. 3, pp. 658–665, 1999.
- P. Li and T. E. Martin, “Nest-site selection and nesting success of cavity-nesting birds in high elevation forest drainages,” The Auk, vol. 108, no. 2, pp. 405–418, 1991.
- B. Schreiber and D. S. deCalesta, “The relationship between cavity-nesting birds and snags on clearcuts in western Oregon,” Forest Ecology and Management, vol. 50, no. 3-4, pp. 299–316, 1992.
- K. Martin, K. E. H. Aitken, and K. L. Wiebe, “Nest sites and nest webs for cavity-nesting communities in interior British Columbia, Canada: nest characteristics and niche partitioning,” Condor, vol. 106, no. 1, pp. 5–19, 2004.
- K. B. Aubry and C. M. Raley, “Selection of nest and roost trees by pileated woodpeckers in coastal forests of Washington,” Journal of Wildlife Management, vol. 66, no. 2, pp. 392–406, 2002.
- E. L. Bull, “Ecology of the pileated woodpecker in northeastern Oregon,” Journal of Wildlife Management, vol. 51, no. 2, pp. 472–481, 1987.
- P. Ohanjanian, I. A. Manley, and P. Davidson, “Williamson’s sapsucker in the East Kootenay Region of British Columbia: results of 2006 inventory,” Report to Tembec Industries, Forest Investment Account, BC Ministry of Environment, Victoria, Canada, 2006.
- R. van den Driessche, T. Chatwin, and M. Mather, “Habitat selection by bats in temperate old-growth forests, Clayoquot Sound, British Columbia,” in Proceedings of the Conference on the Biology and Management of Species and Habitats at Risk, L. Darling, Ed., vol. 1, pp. 313–319, BC Ministry of Environment, Lands and Parks, University College of the Cariboo, Kamloops, Canada, 2000.
- M. J. Vonhof and J. C. Gwilliam, “Intra- and interspecific patterns of day roost selection by three species of forest-dwelling bats in Southern British Columbia,” Forest Ecology and Management, vol. 252, no. 1–3, pp. 165–175, 2007.
- C. L. Mahon, J. D. Steventon, and K. Martin, “Cavity and bark nesting bird response to partial cutting in Northern conifer forests,” Forest Ecology and Management, vol. 256, no. 12, pp. 2145–2153, 2008.
- D. E. Runde and D. E. Capen, “Characteristics of northern hardwood trees used by cavity-nesting birds.,” Journal of Wildlife Management, vol. 51, no. 1, pp. 217–223, 1987.
- C. Savignac and C. S. Machtans, “Habitat requirements of the Yellow-bellied Sapsucker, Sphyrapicus varius, in boreal mixedwood forests of northwestern Canada,” Canadian Journal of Zoology, vol. 84, no. 9, pp. 1230–1239, 2006.
- N. Nielsen-Pincus and E. O. Garton, “Responses of cavity-nesting birds to changes in available habitat reveal underlying determinants of nest selection,” Northwest Naturalist, vol. 88, no. 3, pp. 135–146, 2007.
- J. A. Deal and M. Setterington, Woodpecker Nest Habitat in the Nimpkish Valley, Northern Vancouver Island, Canadian Forest Products Ltd., Woss, Canada, 2000.
- British Columbia Nest Records Scheme, Biodiversity Centre of Wildlife Studies, Victoria, Canada.
- R. T. Reynolds, B. D. Linkhart, and J. J. Jeanson, Characteristics of Snags and Trees Containing Cavities in a Colorado Conifer Forest, USDA Forest Service Research Note RM-RN-455, 1985.
- W. Klenner and D. J. Huggard, “Nesting and foraging habitat requirements of woodpeckers in relation to experimental harvesting treatments at Opax Mountain,” in Proceedings of the Dry Douglas-Fir Workshop New Information for the Management of Dry Douglas-Fir Forests, C. Hollstedt, A. Vyse, and D. Huggard, Eds., pp. 277–291, Research Branch, BC Ministry of Forests, 1998.
- S. J. Madsen, Habitat use by cavity-nesting birds in the Okanogan National Forest, Washington [M.S. thesis], University of Washington, Seattle, Wash, USA, 1985.
- M. G. Raphael and M. White, “Use of snags by cavity-nesting birds in the Sierra Nevada,” Wildlife Monographs, no. 86, pp. 1–66, 1984.
- B. Winternitz and H. Cahn, “Nestholes in live and dead aspen,” in Proceedings of the Symposium in Snag Habitat Management, J. W. Davis, G. A. Goodwin, and R. A. Ockenfels, Eds., pp. 102–106, USDA Forest Service General Technical Report RM-99, 1983.
- R. W. Lundquist and J. M. Mariani, “Nesting habitat and abundance of snag-dependent birds in the southern Washington Cascade Range,” in Wildlife and Vegetation of Unmanaged Douglas-Fir Forest, L. F. Ruggiero, K. B. Aubry, A. B. Carey, and M. H. Huff, Eds., pp. 220–239, USDA Forest Service General Technical Report PNW-286, 1991.
- E. Miller and D. R. Miller, “Snag use by birds,” in Management of Western Forests and Grasslands for Nongame Birds, R. M. DeGraaf, Ed., pp. 337–356, USDA Forest Service General Technical Report INT-GTR-86, 1980.
- E. L. Bull, Resource partitioning among woodpeckers in northeastern Oregon [Ph.D. thesis], University of Idaho, Moscow, Idaho, USA, 1980.
- E. L. Caton, Effects of fire and salvage logging on the cavity-nesting bird community in northwestern Montana [Ph.D. thesis], University of Montana, Missoula, Mont, USA, 1996.
- N. Hoffman, Distribution of Picoides woodpeckers in relation to habitat disturbance within theYellow stone area [M.S. thesis], Montana State University, Bozeman, Mont, USA, 1997.
- R. W. Campbell, N. K. Dawe, I. McTaggart-Cowan, et al., The Birds of British Columbia. Vol. 2, Diurnal Birds of Prey Through Woodpeckers, University of British Columbia Press, Vancouver, Canada, 1990.
- R. N. Conner, R. G. Hooper, H. S. Crawford, et al., “Woodpecker nesting habitat in cut and uncut woodlands in Virginia,” Journal of Wildlife Management, vol. 39, no. 1, pp. 144–150, 1975.
- K. E. Kelleher, A study of hole-nesting avifauna of southwest British Columbia [M.S. thesis], University of British Columbia, Vancouver, Canada, 1963.
- V. E. Scott, J. A. Whelan, and P. L. Svoboda, “Cavity-nesting birds and forest management,” in Proceedings of the Workshop on the Management of Western Forests and Grasslands for Nongame Birds, R. M. DeGraaf, Ed., pp. 311–324, USDA Forest Service General Technical Report INT-GTR-86, 1980.
- K. L. Wiebe, “Microclimate of tree cavity nests: is it important for reproductive success in northern flickers?” Auk, vol. 118, no. 2, pp. 412–421, 2001.
- B. R. McClelland and P. T. McClelland, “Pileated woodpecker nest and roost trees in Montana: links with old-growth and forest “health”,” Wildlife Society Bulletin, vol. 27, no. 3, pp. 846–857, 1999.
- T. K. Mellen, Home range and habitat use of Pileated Woodpeckers, western Oregon [M.S. thesis], Oregon State University, Corvalis, Ore, USA, 1987.
- J. B. Joy, “Characteristics of nest cavities and nest trees of the Red-breasted Sapsucker in Coastal Montane forests,” Journal of Field Ornithology, vol. 71, no. 3, pp. 525–530, 2000.
- B. R. McClelland and P. T. McClelland, “Red-naped Sapsucker nest trees in Northern Rocky Mountain old-growth forest,” Wilson Bulletin, vol. 112, no. 1, pp. 44–50, 2000.
- C. J. Conway and T. E. Martin, “Habitat suitability for Williamsons's sapsuckers in mixed-conifer forests,” Journal of Wildlife Management, vol. 57, no. 2, pp. 322–328, 1993.
- P. N. Hooge, M. T. Stanback, and W. D. Koenig, “Nest-site selection in the acorn woodpecker,” Auk, vol. 116, no. 1, pp. 45–54, 1999.
- R. W. Campbell, N. K. Dawe, I. Mctaggart-Cowan, et al., The Birds of British Columbia. Vol. 3. Passerines: Flycatchers Through Vireos, University of British Columbia Press, Vancouver, Canada, 1997.
- G. K. Peck and R. D. James, Breeding Birds of Ontario, Nidiology and Distribution, Volume 2: Passerines, Royal Ontario Museum, Toronto, Canada, 1987.
- A. H. Miller and C. E. Bock, “Natural history of the Nuttall Woodpecker at the Hastings Reservation,” The Condor, vol. 74, no. 3, pp. 284–294, 1972.
- S. M. McEllin, “Nest sites and population demographies of White-breasted and Pygmy Nuthatches in Colorado,” The Condor, vol. 81, no. 4, pp. 348–352, 1979.
- K. A. Milne and S. J. Hejl, “Nest-site characteristics of white-headed woodpeckers,” Journal of Wildlife Management, vol. 53, no. 1, pp. 50–55, 1989.
- R. D. Dixon, Ecology of White-headed Woodpeckers in the central Oregon Cascades [M.S. thesis], University of Idaho, Moscow, Idaho, USA, 1995.
- M. Haggard and W. L. Gaines, “Effects of stand-replacement fire and salvage logging on a cavity-nesting bird community in eastern Cascades, Washington,” Northwest Science, vol. 75, no. 4, pp. 387–396, 2001.
- S. M. Hitchcock, Abundanceand nesting success of cavity nesting birds in unlogged and salvage-logged burned forests in northwest Montana [M.S. thesis], University of Montana, Missoula, Mont, USA, 1996.
- C. Y. Smith, I. G. Warkentin, and M. T. Moroni, “Snag availability for cavity nesters across a chronosequence of post-harvest landscapes in western Newfoundland,” Forest Ecology and Management, vol. 256, no. 4, pp. 641–647, 2008.
- M. A. Harris, Habitat use among woodpeckers on forest burns [M.S. thesis], University of Montana, Missoula, Mont, USA, 1982.
- J. A. Sedgwick and F. L. Knopf, “Habitat relationships and nest site characteristics of cavity- nesting birds in cottonwood floodplains,” Journal of Wildlife Management, vol. 54, no. 1, pp. 112–124, 1990.
- L. W. Gyug, C. Steeger, and I. Ohanjanian, “Characteristics and densities of Williamson's Sapsucker nest trees in British Columbia,” Canadian Journal of Forest Research, vol. 39, no. 12, pp. 2319–2331, 2009.
- G. C. Daily, “Heartwood decay and vertical distribution of red-naped sapsucker nest cavities,” Wilson Bulletin, vol. 105, no. 4, pp. 674–679, 1993.
- C. J. Conway and T. E. Martin, “Habitat suitability for Williamsons's sapsuckers in mixed-conifer forests,” Journal of Wildlife Management, vol. 57, no. 2, pp. 322–328, 1993.
- L. Saari, E. Pulliainen, O. Hildén, A. Järvinen, and I. Mäkisalo, “Breeding biology of the Siberian Tit Parus cinctus in Finland,” Journal of Ornithology, vol. 135, no. 4, pp. 549–575, 1994.
- D. G. Keisker, “Nest tree selection by primary cavity-nesting birds in south-central British Columbia,” Wildlife Report R-13, BC Ministry of Environment, Lands and Parks, Victoria, Canada, 1987.
- K. M. Mazur, P. C. James, and S. D. Frith, “Barred Owl (Strix varia) nest site characteristics in the boreal forest of Saskatchewan,” in Proceedings of the 2nd International Symposium in Biology and Conservation of Owls of the Northern Hemisphere, J. R. Duncan, D. H. Johnson, and T. H. Nicholls, Eds., pp. 267–271, USDA Forest Service, General Technical Report NC-190, 1997.
- M. R. Evans, Breeding habitat selection by Barrow’s Goldeneye and Bufflehead in the Cariboo-Chilcotin region of British Columbia: nest sites, brood-rearing habitat, and competition [Ph.D. thesis], Simon Fraser University, Burnaby, Canada, 2003.
- C. M. Davis, “A nesting study of the Brown Creeper,” Living Bird, vol. 17, pp. 237–263, 1978.
- J. J. Siegel, An evaluation of the minimum habitat quality standards for birds in old-growthponderosa pine forests, northern Arizona [M.S. thesis], University of Arizona, Tucson, Ariz, USA, 1989.
- C. Steeger and J. Dulisse, “Ecological interrelationships of three-toed woodpeckers with bark beetles and pine trees,” in Research Summary RS-035, pp. 1–4, BC Ministry of Forests, Nelson, Canada, 1997.
- M. A. Stern, T. G. Wise, and K. L. Theodore, “Use of natural cavity by bufflehead nesting in Oregon,” The Murrelet, vol. 68, no. 2, p. 50, 1987.
- D. P. Arsenault, “Differentiating nest sites of primary and secondary cavity-nesting birds in New Mexico,” Journal of Field Ornithology, vol. 75, no. 3, pp. 257–265, 2004.
- R. W. Campbell, N. K. Dawe, I. McTaggart-Cowan, et al., “The birds of British Columbia,” in Introduction, Loons Through Waterfowl, vol. 1, University of British Columbia Press, Vancouver, Canada, 1990.
- D. F. Stauffer and L. B. Best, “Nest-site selection by cavity-nesting birds of riparian habitats in Iowa.,” Wilson Bulletin, vol. 94, no. 3, pp. 329–337, 1982.
- W. B. Rendell and R. J. Robertson, “Nest-site characteristics, reproductive success and cavity availability for Tree Swallows breeding in natural cavities,” The Condor, vol. 91, no. 4, pp. 875–885, 1989.
- E. L. Bull and C. T. Collins, “Nest site fidelity, breeding age, and adult longevity in the Vaux's swift,” North American Bird Bander, vol. 21, no. 2, pp. 49–51, 1996.
- E. L. Bull and H. D. Cooper, “Vaux's swift nests in hollow trees,” Western Birds, vol. 22, no. 2, pp. 85–91, 1991.
- J. E. Hunter and M. J. Mazurek, “Characteristics of trees used by nesting and roosting Vaux's Swifts in northwestern California,” Western Birds, vol. 34, no. 4, pp. 225–229, 2003.
- G. D. Hayward, P. H. Hayward, and E. O. Garton, “Ecology of boreal owls in the northern Rocky Mountains, USA,” Wildlife Monographs, no. 124, pp. 1–59, 1993.
- J. B. Cunningham, R. P. Balda, and W. S. Gaud, “Selection and use of snags by secondary and cavity-nesting birds of the ponderosa pine forest,” USDA Forest Service Research Paper RM-RP-222, 1980.
- J. R. Robb and T. A. Bookhout, “Factors influencing wood duck use of natural cavities,” Journal of Wildlife Management, vol. 59, no. 2, pp. 372–383, 1995.
- D. S. Gilmer, I. J. Ball, L. M. Cowardin, et al., “Natural cavities used by wood ducks in north-central Minnesota,” Journal of Wildlife Management, vol. 42, no. 2, pp. 288–298, 1978.
- G. J. Soulliere, “Density of suitable wood duck nest cavities in a northern hardwood forest,” Journal of Wildlife Management, vol. 52, no. 1, pp. 86–89, 1988.
- H. H. Prince, “Nest sites used by wood ducks and common goldeneyes in New Brunswick,” Journal of Wildlife Management, vol. 32, no. 3, pp. 489–500, 1968.
- L. L. C. Jones, M. G. Raphael, J. T. Forbes, et al., “Using remotely activated cameras to monitor maternal dens of martens,” in Martes: Taxonomy, Ecology, Techniques, and Management, G. Proulx, H. N. Bryant, and P. M. Woodard, Eds., pp. 329–349, Provincial Museum of Alberta, Edmonton, Canada, 1997.
- S. K. Martin and R. H. Barrett, “Resting site selection by marten at Sagehen Creek, California,” Northwestern Naturalist, vol. 72, no. 2, pp. 37–42, 1991.
- M. G. Raphael and L. L. C. Jones, “Characteristics of resting and denning sites of American martens in central Oregon and western Washington,” in Martes: Taxonomy, Ecology, Techniques and Management, G. Proulx, H. N. Bryant, and P. M. Woodward, Eds., pp. 146–166, Provincial Museum of Alberta, Edmonton, Canada, 1997.
- L. F. Ruggiero, D. E. Pearson, and S. E. Henry, “Characteristics of American marten den sites in Wyoming,” Journal of Wildlife Management, vol. 62, no. 2, pp. 663–673, 1998.
- W. D. Spencer, “Seasonal rest-site preferences of pine martens in the northern Sierra Nevada,” Journal of Wildlife Management, vol. 51, no. 3, pp. 616–621, 1987.
- B. J. Betts, “Roosting behaviour of silver-haired bats (Lasionycteris noctivagans) and big brown bats (Eptesicus fuscus) in northeast Oregon,” in Proceedings of the Bats and Forests Symposium, R. M. R. Barclay and R. M. Brigham, Eds., pp. 61–66, British Columbia Ministry of Forests, 1996.
- M. J. Rabe, T. E. Morrell, and H. Green, “Characteristics of ponderosa pine snag roosts used by reproductive bats in northern Arizona,” Journal of Wildlife Management, vol. 62, no. 2, pp. 612–621, 1998.
- S. A. Rasheed and S. L. Holroyd, Roosting Habitat Assessment and Inventory of Bats in the Mica Wildlife Compensation Area, Columbia Basin Fish and Wildlife Compensation Program, Nelson, Canada, 1995.
- M. J. Vonhof, A Survey of the Abundance, Diversity, and Roost-Site Preferences of Bats in the Pend d'Oreille Valley, British Columbia, Columbia Basin Fish and Wildlife Compensation Program, Nelson, Canada, 1996.
- J. Akenson, Black Bear Den Summary NE Region—Interim Progress Report. Starkey Bear Study, Oregon Department of Fish and Game, La Grande, Ore, USA, 1994.
- E. L. Bull, J. J. Akenson, B. J. Betts, et al., “The interdependence of wildlife and old-growth forests,” in Proceedings of the Workshop on Wildlife Tree/Stand-Level Biodiversity, P. Bradford, T. Manning, and B. I'Anson, Eds., pp. 71–76, BC Ministry of Environment, Lands, and Parks and Ministry of Forests, 1996.
- H. Davis, Characteristics and selection of winter dens by black bears in coastal British Columbia [M.S. thesis], Simon Fraser University, Burnaby, Canada, 1996.
- D. Immell and M. C. Boulay, Progress Report—Black Bear Ecology Research Project. Wildlife Research Project, Oregon Department of Fish and Wildlife, Portland, Ore, USA, 1994.
- D. J. Lindsay, Black Bear den Catalogue: A Listing of Coastal Black Bear Dens, Timberwest Forest, Crofton, Canada, 1999.
- F. G. Lindzey and C. Meslow, “Characteristics of black bear dens on Long Island, Washington,” Northwest Science, vol. 60, no. 4, pp. 236–242, 1976.
- W. O. Noble, C. E. Meslow, and M. D. Pope, Denning Habits of Black Bears in the Central Coast Range of Oregon, Department of Fisheries and Wildlife, Oregon State University, Corvallis, Ore, USA, 1990.
- R. M. Brigham, M. J. Vonhof, R. M. R. Barclay, and J. C. Gwilliam, “Roosting behavior and roost-site preferences of forest-dwelling california bats (Myotis californicus),” Journal of Mammalogy, vol. 78, no. 4, pp. 1231–1239, 1997.
- S. Grindal, Upper Kootenay River Bat Survey, Columbia Basin Fish and Wildlife Compensation Program, Nelson, Canada, 1997.
- T. F. Paragi, S. M. Arthur, and W. B. Krohn, “Importance of tree cavities as natal dens for fishers,” Northern Journal of Applied Forestry, vol. 13, no. 2, pp. 79–83, 1996.
- J. S. Yaeger, Habitat at fisher resting sites in the Klamath province of northern California [M.S. thesis], Humboldt State University, Arcata, Calif, USA, 2005.
- A. B. Carey, T. M. Wilson, C. C. Maguire, and B. L. Biswell, “Dens of northern flying squirrels in the pacific northwest,” Journal of Wildlife Management, vol. 61, no. 3, pp. 684–699, 1997.
- R. A. Mowrey and J. C. Zasada, “Den tree use and movements of northern flying squirrels in interior Alaska and implications for forest management,” in Proceedings of the Symposium on Fish and Wildlife Relationships in Old-Growth Forests, W. R. Meehan, T. R. Merrell, and T. A. Hanley, Eds., pp. 351–356, American Institute of Fishery Research Biologists, 1984.
- C. Steeger and J. Dulisse, “Characteristics and dynamics of cavity nest trees in southern British Columbia,” in Proceedings of the Symposium on the Ecology and Management of Dead Wood in Western Forests, W. F. Laudenslayer Jr., P. J. Shea, B. E. Valentine, C. P. Weatherspoon, and T. E. Lisle, Eds., pp. 275–289, USDA Forest Service, General Technical Report PSW-GTR-181, 2002.
- T. J. Weller and C. J. Zabel, “Characteristics of fringed myotis day roosts in northern California,” Journal of Wildlife Management, vol. 65, no. 3, pp. 489–497, 2001.
- L. H. Crampton and R. M. R. Barclay, “Relationships between bats and stand age and structure in aspen mixedwood forests in Alberta,” in Relationships Between Stand Age, Stand Structure and Biodiversity in Aspen Mixedwood Forests in Alberta, B. Stelfox, Ed., pp. 211–225, Alberta Environmental Centre, Vegreville, Canada; Canadian Forest Service, Edmonton, Canada, 1995.
- M. C. Kalcounis and K. R. Hecker, “Intraspecific variation in roost-site selection by little brown bats (Myotis lucifugus),” in Proceedings of the Bats and Forests Symposium, R. M. R. Barclay and R. M. Brigham, Eds., pp. 81–90, British Columbia Ministry of Forests, 1996.
- P. C. Ormsbee and W. C. McComb, “Selection of day roosts by female long-legged myotis in the central Oregon Cascade Range,” Journal of Wildlife Management, vol. 62, no. 2, pp. 596–603, 1998.
- M. C. Caceres, The summer ecology of Myotis species bats in the interior wet-belt of British Columbia [M.S. thesis], University of Calgary, Calgary, Canada, 1998.
- M. A. Menzel, S. F. Owen, W. M. Ford et al., “Roost tree selection by northern long-eared bat (Myotis septentrionalis) maternity colonies in an industrial forest of the central Appalachian mountains,” Forest Ecology and Management, vol. 155, no. 1-3, pp. 107–114, 2002.
- D. L. Waldien, J. P. Hayes, and E. B. Arnett, “Day-roosts of female long-eared myotis in western Oregon,” Journal of Wildlife Management, vol. 64, no. 3, pp. 785–796, 2000.
- L. A. Campbell, J. G. Hallett, and M. A. O'Connell, “Conservation of bats in managed forests: use of roosts by Lasionycteris noctivagans,” Journal of Mammalogy, vol. 77, no. 4, pp. 976–984, 1996.
- M. J. Vonhof and R. M. R. Barclay, “Use of tree stumps as roosts by the western long-eared bat,” Journal of Wildlife Management, vol. 61, no. 3, pp. 674–684, 1997.
- P. L. Svoboda, K. E. Young, and V. E. Scott, “Recent nesting records of Purple Martins in western Colorado,” Western Birds, vol. 11, no. 4, pp. 195–198, 1980.
- A. D. M. Rayner and L. Boddy, Fungal Decomposition of Wood, John Wiley and Sons, Chichester, UK, 1988.
- F. L. Bunnell and A. C. Chan-McLeod, “Terrestrial vertebrates,” in The Rain Forests of Home. Profile of a North American Bioregion, P. K. Schoonmaker, B. von Hagen, and E. C. Wolf, Eds., pp. 103–130, Island Press, Washington, DC, USA, 1997.
- J. J. Akenson and M. G. Henjum, “Black bear den site selection in the Starkey study area,” Blue Mountains Natural Resources Institute, Natural Resource News, vol. 4, no. 2, pp. 1–2, 1994.
- A. T. Hamilton, Personal Communication, BC Ministry of Fish and Wildlife, Victoria, Canada, 2010.
- W. C. McComb and R. E. Noble, “Nest-box and natural-cavity use in three mid-south forest habitats,” Journal of Wildlife Management, vol. 445, no. 1, pp. 93–101, 1981.
- S. J. Hejl, K. R. Newlon, M. E. Mcfadzen, et al., “Brown Creeper (Certhia americana),” in The Birds of North America Online, A. Poole, Ed., Cornell Lab of Ornithology, Ithaca, NY, USA, 2002.
- B. Peterson and G. Gauthier, “Nest site use by cavity-nesting birds of the Cariboo Parkland, British Columbia,” The Wilson Bulletin, vol. 97, no. 3, pp. 319–331, 1985.
- F. L. Bunnell, E. Wind, and R. Wells, “Dying and dead hardwoods: their implications to management,” in Proceedings of the Symposium on the Ecology and Management of Dead Wood in Western Forests, W. F. Laudenslayer Jr, P. J. Shea, B. E. Valentine, C. P. Weatherspoon, and T. E. Lisle, Eds., pp. 695–716, USDA Forest Service, General Technical Report PSW-GTR-181, 2002.
- R. Goggans, R. D. Dixon, and L. C. Seminara, “Habitat use by three-toed and black- backed woodpeckers, Deschutes National Forest,” USDA Forest Service Technical Report 87-3-02, 1989.
- W. Klenner and D. Huggard, “Three-toed woodpecker nesting and foraging at Sicamous Creek,” in Proceedings of the Workshop on the Sicamous Creek Silvicultural Systems Project, C. Hollstedt and A. Vyse, Eds., pp. 224–233, Research Branch, BC Ministry of Forests, 1997.
- E. L. Bull and J. A. Jackson, “Pileated woodpecker (Dryocopus pileatus),” in The Birds of North America Online, A. Poole, Ed., Cornell Lab of Ornithology, Ithaca, NY, USA, 2011.
- A. B. Crockett and H. H. Hadow, “Nest site selection by Williamson's and red-naped sapsuckers,” The Condor, vol. 77, no. 3, pp. 365–368, 1975.
- M. Axelrod, “Observations on a boreal chickadee nest,” The Loon, vol. 51, pp. 135–140, 1979.
- C. Galen, “A preliminary assessment of the status of the Lewis' Woodpecker in Wasco County, Oregon,” Tech. Rep. 88-3-01, Oregon Department of Fish and Wildlife, Portland, Ore, USA, 1989.
- B. P. Booth, The effects of thinning on forest bird communities in dry, interior Douglas-fir forests [M.S. thesis], University of British Columbia, Vancouver, Canada.
- K. L. Garrett, M. G. Raphael, and R. D. Dixon, “White-headed Woodpecker (Picoides albolarvatus),” in The Birds of North America Online, A. Poole, Ed., Cornell Lab of Ornithology, Ithaca, NY, USA, 1996.
- W. C. Weber and S. R. Cannings, “The white-headed woodpecker (Dendrocopus albovatus) In British Columbia,” Syesis, vol. 9, pp. 215–220, 1976.
- C. L. Hartwig, D. S. Eastman, and A. S. Harestad, “Characteristics of pileated woodpecker (Dryocopus pileatus) cavity trees and their patches on southeastern Vancouver Island, British Columbia, Canada,” Forest Ecology and Management, vol. 187, no. 2-3, pp. 225–234, 2004.
- C. Steeger, M. Machmer, and E. Walters, “Ecology and management of woodpeckers and wildlife trees in British Columbia,” in Fraser River Action Plan, pp. 1–23, Canadian Wildlife Service, Delta, Canada, 1996.
- R. L. Hutto and S. M. Gallo, “The effects of postfire salvage logging on cavity-nesting birds,” Condor, vol. 108, no. 4, pp. 817–831, 2006.
- R. A. Cannings, R. J. Cannings, and S. G. Cannings, Birds of the Okanagan Valley, British Columbia, Royal British Columbia Museum, Victoria, Canada, 1987.
- J. A. Deal and D. W. Gilmore, “Effects of vertical structure and biogeoclimatic subzone on nesting locations for woodpeckers on north central Vancouver Island: nest tree attributes,” Northwest Science, vol. 72, no. 2, pp. 119–121, 1998.
- S. Hågvar, G. Hågvar, and E. Mønness, “Nest site selection in Norwegian woodpeckers,” Holarctic Ecology, vol. 13, no. 2, pp. 156–165, 1990.
- T. Wesołowski and L. Tomiałojć, “The breeding ecology of woodpeckers in a temperate primaeval forest—preliminary data,” Acta Ornithologica, vol. 22, no. 1, pp. 1–21, 1986.
- K. Eckert, “First Minnesota nesting record of northern three-toed woodpecker,” Loon, vol. 53, pp. 221–223, 1981.
- A. J. Erskine and W. D. McLaren, “Sapsucker nest holes and their use by other species,” The Canadian Field-Naturalist, vol. 86, no. 4, pp. 357–361, 1972.
- C. E. Bock, “The ecology and behavior of the Lewis's Woodpecker (Asyndesmus lewis),” in University of California Publication in Zoology, vol. 92, University of California Press, Berkeley, Calif, USA, 1970.
- W. M. Block, “Foraging ecology of Nuttall's woodpecker,” The Auk, vol. 108, no. 2, pp. 303–318, 1991.
- L. L. Short Jr., “The systematics and behavior of some North American woodpeckers, genus Picoides(Aves),” Bulletin of the American Museum of Natural History, vol. 145, 118 pages, 1971.
- J. R. Waters, Population and habitat characteristics of cavity-nesting birds in a California oak woodland [M.S. thesis], Humboldt State University, Arcata, Calif, USA, 1988.
- V. A. Saab and J. G. Dudley, Responses of Cavity-Nesting Birds to Stand-Replacement Fire and Salvage Logging in Ponderosa Pine/Douglas-Fir Forests of Southwestern Idaho, USDA Forest Service Research Paper RMRS-RP-11, 1998.
- D. L. Leonard Jr., “Three-toed Woodpecker (Picoides tridactylus),” in The Birds of North America Online, A. Poole, Ed., Cornell Lab of Ornithology, Ithaca, NY, USA, 2001.
- R. W. Mannan, E. C. Meslow, and H. M. Wight, “Use of snags by birds in Douglas-fir forests, Western Oregon,” Journal of Wildlife Management, vol. 44, no. 4, pp. 787–797, 1980.
- B. G. Marcot and R. Hill, “Flammulated owls in northwestern California,” Western Birds, vol. 11, no. 3, pp. 141–149, 1980.
- M. L. Richmond, L. R. DeWeese, and R. E. Pillmore, “Brief observations on the breeding biology of the flammulated owl in Colorado,” Western Birds, vol. 11, no. 1, pp. 35–46, 1980.
- B. Webb, “Distribution and nesting requirements of montane forest owls in Colorado—part III: flammulated owl (Otus flammeolus),” Journal of the Colorado Field Ornithologists, vol. 6, pp. 76–81, 1982.
- E. L. Bull, A. L. Wright, and M. G. Henjum, “Nesting habitat of flammulated owls in Oregon,” Journal of Raptor Research, vol. 24, no. 3, pp. 52–55, 1990.
- R. T. Reynolds and B. D. Linkhart, “The nesting biology of flammulated owls in Colorado,” in Proceedings of the Biology and Conservation of Northern Forest Owls Symposium, R. W. Nero, R. J. Clark, R. J. Knapton, and R. H. Hamre, Eds., pp. 239–248, USDA Forest Service Technical Report RM-GTR-42, 1997.
- A. M. van Woudenberg, Integrated management of flammulated owl breeding habitat and timber harvest in British Columbia [M.S. thesis], University of British Columbia, Vancouver, Canada, 1992.
- H. E. Kingery and C. K. Ghalambor, “Pygmy nuthatch (Sitta pygmaea),” in The Birds of North America Online, A. Poole, Ed., Cornell Lab of Ornithology, Ithaca, NY, USA, 2001.
- T. Brush, B. W. Anderson, and R. D. Ohmart, “Habitat selection related to resource availability among cavity-nesting birds,” in Proceedings of the Snag Habitat Management Symposium, J. W. Davis and G. A. Goodwin R .A. Ockenfels, Eds., pp. 88–98, USDA Forest Service General Technical Report RM-99, 1983.
- S. P. Cline, A. B. Berg, and H. M. Wight, “Snag characteristics and dynamics in Douglas-fir forests, western Oregon,” Journal of Wildlife Management, vol. 44, no. 4, pp. 773–786, 1980.
- K. H. Wright and G. M. Harvey, The Deterioration of Beetle-Killed Douglas-Fir in Western Oregon and Washington, USDA Forest Service Research Paper PNW-RP-50, 1967.
- C. Steeger and C. L. Hitchcock, “Influence of forest structure and diseases on nestsite selection by red-breasted nuthatches,” Journal of Wildlife Management, vol. 62, no. 4, pp. 1349–1358, 1998.
- T. S. Buchanan and G. H. Englerth, Decay and Other Losses in Windthrown Timber on the Olympic Peninsula, Washington, USDA Forest Service Technical Bulletin 733, Washington, DC, USA, 1940.
- N. T. Engelhardt, “Pathological deterioration of looper-killed western hemlock on southern Vancouver Island,” Forest Science, vol. 3, no. 2, pp. 125–136, 1957.
- S. Parsons, K. J. Lewis, and J. M. Psyllakis, “Relationships between roosting habitat of bats and decay of aspen in the sub-boreal forests of British Columbia,” Forest Ecology and Management, vol. 177, no. 1–3, pp. 559–570, 2003.
- P. M. Cryan, M. A. Bogan, and G. M. Yanega, “Roosting habits of four bat species in the black hills of South Dakota,” Acta Chiropterologica, vol. 3, no. 1, pp. 43–52, 2001.
- R. W. Perry and R. E. Thill, “Roost selection by big brown bats in forests of Arkansas: importance of Pine snags sand open forest habitats to males,” Southeastern Naturalist, vol. 7, no. 4, pp. 607–618, 2008.
- S. J. Rancourt, M. I. Rule, and M. A. O'Connell, “Maternity roost site selection of big brown bats in ponderosa pine forests of the Channeled Scablands of northeastern Washington State, USA,” Forest Ecology and Management, vol. 248, no. 3, pp. 183–192, 2007.
- R. M. R. Barclay and R. M. Brigham, “Year-to-year reuse of tree-roosts by California bats (Myotis californicus) in southern British Columbia,” American Midland Naturalist, vol. 146, no. 1, pp. 80–85, 2001.
- J. L. Boland, J. P. Hayes, W. P. Smith, and M. M. Huso, “Selection of day-roosts by Keen's myotis (Myotis Keenii) at multiple spatial scales,” Journal of Mammalogy, vol. 90, no. 1, pp. 222–234, 2009.
- L. H. Crampton and R. M. R. Barclay, “Selection of roosting and foraging habitat by bats in different-aged aspen mixedwood stands,” Conservation Biology, vol. 12, no. 6, pp. 1347–1358, 1998.
- J. M. Psyllakis and R. M. Brigham, “Characteristics of diurnal roosts used by female Myotis bats in sub-boreal forests,” Forest Ecology and Management, vol. 223, no. 1–3, pp. 93–102, 2006.
- P. C. Ormsbee, “Characteristics, use, and distribution of day roosts selected by female Myotis volans (long-legged myotis) in forested habitat of the central Oregon Cascades,” in Proceedings of the Bats and Forests Symposium, R. M. R. Barclay and R. M. Brigham, Eds., pp. 124–130, British Columbia Ministry of Forests, 1996.
- R. W. Foster and A. Kurta, “Roosting ecology of the northern bat (Myotis septentrionalis) and comparisons with the endangered Indiana bat (Myotis sodalis),” Journal of Mammalogy, vol. 80, no. 2, pp. 659–672, 1999.
- M. J. Lacki and J. H. Schwierjohann, “Day-roost characteristics of northern bats in mixed mesophytic forest,” Journal of Wildlife Management, vol. 65, no. 3, pp. 482–488, 2001.
- M. D. Baker, M. J. Lacki, G. A. Faixa, P. L. Droppelman, R. A. Slack, and S. A. Slankard, “Habitat use of pallid bats in coniferous forests of northern California,” Northwest Science, vol. 82, no. 4, pp. 269–275, 2008.
- R. M. R. Barclay, P. A. Faure, and D. R. Farr, “Roosting behavior and roost selection by migrating silver-haired bats (Lasionycteris noctivagans),” Journal of Mammalogy, vol. 69, no. 4, pp. 821–825, 1988.
- T. A. Mattson, S. W. Buskirk, and N. L. Stanton, “Roost sites of the silver-haired bat (Lasionycteris noctivagans) in the Black Hills, South Dakota,” Great Basin Naturalist, vol. 56, no. 3, pp. 247–253, 1996.
- C. L. Cotton and K. L. Parker, “Winter habitat and nest trees used by northern flying squirrels in subboreal forests,” Journal of Mammalogy, vol. 81, no. 4, pp. 1071–1086, 2000.
- J. S. Gerrow, Home range, habitat use, nesting ecology and diet of the northern flying squirrel in southern New Brunswick [M.S. thesis], Acadia University, Wolfville, Canada, 1996.
- G. L. Holloway and J. R. Malcolm, “Nest-tree use by northern and southern flying squirrels in central Ontario,” Journal of Mammalogy, vol. 88, no. 1, pp. 226–233, 2007.
- K. J. Martin, Movements and habitat characteristics of northern flying squirrels in the central Oregon Cascades [M.S. thesis], Oregon State University, Corvallis, Ore, USA, 1994.
- M. D. Meyer, D. A. Kelt, and M. P. North, “Nest trees of northern flying squirrels in the Sierra Nevada,” Journal of Mammalogy, vol. 86, no. 2, pp. 275–280, 2005.
- M. D. Meyer, M. P. North, and D. A. Kelt, “Nest trees of northern flying squirrels in Yosemite National Park, California,” Southwestern Naturalist, vol. 52, no. 1, pp. 157–161, 2007.
- J. W. Witt, “Home range and density estimates for the northern flying squirrel, Glaucomys sabrinus, in western Oregon,” Journal of Mammalogy, vol. 73, no. 4, pp. 921–929, 1992.
- M. J. Lacki and M. D. Baker, “Day roosts of female fringed myotis (Myotis thysanodes) in xeric forests of the pacific northwest,” Journal of Mammalogy, vol. 88, no. 4, pp. 967–973, 2007.
- C. R. Willis and R. M. Brigham, “Physiological and ecological aspects of roost selection by reproductive female hoary bats (Lasiurus cinereus),” Journal of Mammalogy, vol. 86, no. 1, pp. 86–94, 2005.
- D. G. Constantine, “Ecological observations on lasiurine bats in Iowa,” Journal of Mammalogy, vol. 47, no. 1, pp. 34–41, 1966.
- M. A. Vonhof and J. C. Gwilliam, A Summary of Bat Research in the Pend d'Oreille Valley in Southern British Columbia, Columbia Basin Fish and Wildlife Compensation Program, Nelson, Canada, 2000.
- D. I. Solick and R. M. R. Barclay, “Thermoregulation and roosting behaviour of reproductive and nonreproductive female western long-eared bats (Myotis evotis) in the Rocky Mountains of Alberta,” Canadian Journal of Zoology, vol. 84, no. 4, pp. 589–599, 2006.
- S. J. Rancourt, M. I. Rule, and M. A. O'Connell, “Maternity roost site selection of long-eared myotis, Myotis evotis,” Journal of Mammalogy, vol. 86, no. 1, pp. 77–84, 2005.
- M. J. Vonhof and J. C. Gwilliam, Survey of the Roost-Site Preferences of California, Western Long-Eared, and Long-Legged Bats in the Pend d'Oreille Valley, British Columbia, Columbia Basin Fish and Wildlife Compensation Program, Nelson, Canada, 1999.
- R. H. Waring and J. F. Franklin, “Evergreen coniferous forests of the Pacific Northwest,” Science, vol. 204, no. 4400, pp. 1380–1386, 1979.
- S. S. Niemiec, G. R. Ahrens, S. Willits, et al., Hardwoods of the Pacific Northwest, Research Contribution No. 8, Forest Research Laboratory, Oregon State University, Corvallis, Ore, USA, 1995.
- F. L. Bunnell, E. Wind, and M. Boyland, “Diameters and heights of trees with cavities: their implications to management,” in Proceedings of the Symposium on the Ecology and Management of Dead Wood in Western Forests, W. F. Laudenslayer Jr., P. J. Shea, B. E. Valentine, C. P. Weatherspoon, and T. E. Lisle, Eds., pp. 717–738, USDA Forest Service, General Technical Report PSW-GTR-181, 2002.
- F. L. Bunnell, L. L. Kremsater, and E. Wind, “Managing to sustain vertebrate richness in forests of the Pacific Northwest: relationships within stands,” Environmental Reviews, vol. 7, no. 3, pp. 97–146, 1999.
- D. A. Sibley, The Sibley Field Guide to Birds of Western North America, Alfred A. Knopf, New York, NY, USA, 2003.
- B. J. Putnam, Songbird responses of precommercially thinned and unthinned stands in east- central Washington [M.S. thesis], Oregon State University, Corvallis, Ore, US, 1983.
- B. R. McClelland, Relationships between hole-nesting birds, forest snags, and decay in Western larch-douglas-fir forests of the Northern Rocky Mountains [M.S. thesis], University of Montana, Missoula, Mont, USA, 1977.
- B. R. McClelland, S. S. Frissle, W. C. Fischer, et al., “Habitat management for hole-nesting birds in forests of western larch and Douglas-fir,” Journal of Forestry, vol. 77, no. 8, pp. 480–483, 1979.
- S. K. Nelson, Habitat use and densities of cavity-nesting birds in the Oregon coast ranges [M.S. thesis], Oregon State University, Corvallis, Ore, USA, 1988.
- J. E. Zarnowitz and D. A. Manuwal, “The effects of forest management on cavity-nesting birds in northwestern Washington.,” Journal of Wildlife Management, vol. 49, no. 1, pp. 255–263, 1985.
- M. A. Machmer and B. Korol, “Assessment of wildlife tree habitat in the Revelstoke Forest District,” Final Technical Report, Forest Renewal BC, Victoria, Canada, 1998.
- E. L. Bull and C. Meslow, “Habitat requirements of the pileated woodpecker in northeastern Oregon,” Journal of Forestry, vol. 75, no. 6, pp. 335–337, 1977.
- E. L. Bull, R. S. Holthausen, and M. G. Henjum, “Roost trees used by pileated woodpeckers in northeastern Oregon,” Journal of Wildlife Management, vol. 56, no. 4, pp. 786–793, 1992.
- E. L. Walters, Habitat and space use of red-naped sapsucker, Sphyrapicus nuchalis, in the Hat Creek valley, south-central British Columbia [M.S. thesis], University of Victoria, Victoria, Canada, 1990.
- C. L. Mahon, K. Martin, and J. D. Steventon, “Habitat attributes and chestnut-backed chickadee nest site selection in uncut and partial-cut forests,” Canadian Journal of Forest Research, vol. 37, no. 7, pp. 1272–1285, 2007.
- K. A. Linder, Habitat utilization and behavior of nesting Lewis's Woodpeckers (Melanerpes lewis) in the Laramie range, southeast Wyoming [M.S. thesis], University of Wyoming, Laramie, Wyo, USA, 1994.
- K. T. Vierling, “Habitat selection of Lewis' woodpeckers in southeastern Colorado,” Wilson Bulletin, vol. 109, no. 1, pp. 121–130, 1997.
- V. A. Saab, R. E. Russell, and J. G. Dudley, “Nest-site selection by cavity-nesting birds in relation to postfire salvage logging,” Forest Ecology and Management, vol. 257, no. 1, pp. 151–159, 2009.
- R. W. Mannan and E. C. Meslow, “Bird populations and vegetation characteristics in managed and old-growth forests, northeastern Oregon,” Journal of Wildlife Management, vol. 48, no. 4, pp. 1219–1238, 1984.
- K. Viste-Sparkman, White-breasted nuthatch density and nesting ecology in oak woodlands of the Willamette Valley, Oregon [M.S. thesis], Oregon State University, Corvallis, Ore, USA, 2005.
- R. D. Dixon, “Density, nest-site and roost-site characteristics, home-range, habitat-use, and behavior of White-headed Woodpeckers: Deshutes and Winema National Forests, Oregon,” in Nongame Project 93-3-01, pp. 1–90, Oregon Department of Fish and Wildlife, Salem, Ore, USA, 1995.
- B. Fall, “Early summer warbler records and boreal chickadee nest near Itasca Park,” The Loon, vol. 49, pp. 198–201, 1977.
- D. B. Hay and M. Guntert, “Seasonal selection of tree cavities by pygmy nuthatches based on cavity characteristics,” in Proceedings of the Symposium on Snag Habitat Management, J. W. Davis, G. A. Goodwin, and R. A. Ockenfels, Eds., pp. 117–120, USDA Forest Service General Technical Report RM-99, 1983.
- J. F. Poulin, M. A. Villard, M. Edman, P. J. Goulet, and A. M. Eriksson, “Thresholds in nesting habitat requirements of an old forest specialist, the Brown Creeper (Certhia americana), as conservation targets,” Biological Conservation, vol. 141, no. 4, pp. 1129–1137, 2008.
- M. A. Stern, T. G. Wise, and K. L. Theodore, “Use of natural cavity by bufflehead nesting in Oregon,” The Murrelet, vol. 68, no. 2, p. 50, 1987.
- A. P. Yetter, S. P. Havera, and C. S. Hine, “Natural-cavity use by nesting wood ducks in Illinois,” Journal of Wildlife Management, vol. 63, no. 2, pp. 630–638, 1999.
- G. M. Haramis, Wood duck (Aix sponsa) ecology and management within the green-timber impoundments at Montezuma National Wildlife Refuge [M.S. thesis], Cornell University, Ithaca, NY, USA, 1975.
- L. R. Belmonte, Home range and habitat characteristics of boreal owls in northeastern Minnesota [M.S. thesis], University of Minnesota, Duluth, Minn, USA, 2005.
- F. L. Bunnell, L. L. Kremsater, and R. W. Wells, Likely Consequences of Forest Management on Terrestrial, Forest-Dwelling Vertebrates in Oregon, Oregon Forest Resources Institute, Portland, Ore, USA, 1997.
- W. J. Sydeman and M. Guntert, “Winter communal roosting in the pygmy nuthatch,” in Proceedings of the Symposium on Snag Habitat Management, J. W. Davis, G. A. Goodwin, and R. A. Ockenfels, Eds., pp. 121–124, USDA Forest Service General Technical Report RM-99, 1983.
- E. L. Bull and A. K. Blumton, “Roosting behavior of postfledging Vaux's Swifts in northeastern Oregon,” Journal of Field Ornithology, vol. 68, no. 2, pp. 302–305, 1997.
- M. C. Kalcounis-Rüppell, J. M. Psyllakis, and R. M. Brigham, “Tree roost selection by bats: an empirical synthesis using meta-analysis,” Wildlife Society Bulletin, vol. 33, no. 3, pp. 1123–1132, 2005.
- R. M. Brigham, “Flexibility in foraging and roosting behaviour by the big brown bat (Eptesicus fuscus),” Canadian Journal of Zoology, vol. 69, no. 1, pp. 117–121, 1991.
- H. G. Broders and G. J. Forbes, “Interspecific and intersexual variation in roost-site selection of northern long-eared and little brown bats in the greater fundy national park ecosystem,” Journal of Wildlife Management, vol. 68, no. 3, pp. 602–610, 2004.
- C. Caceres, “Northern long-eared bat,” Progress Report 014, Columbia Basin Fish and Wildlife Compensation Program, Nelson, Canada, 1997.
- M. D. Baker and M. J. Lacki, “Day-roosting habitat of female long-legged myotis in ponderosa pine forests,” Journal of Wildlife Management, vol. 70, no. 1, pp. 207–215, 2006.
- T. C. Carter and G. A. Feldhamer, “Roost tree use by maternity colonies of Indiana bats and northern long-eared bats in southern Illinois,” Forest Ecology and Management, vol. 219, no. 2-3, pp. 259–268, 2005.
- C. J. Garroway and H. G. Broders, “Day roost characteristics of northern long-eared bats (Myotis septentrionalis) in relation to female reproductive status,” Ecoscience, vol. 15, no. 1, pp. 89–93, 2008.
- J. B. Johnson, J. W. Edwards, W. M. Ford, and J. E. Gates, “Roost tree selection by northern myotis (Myotis septentrionalis) maternity colonies following prescribed fire in a Central Appalachian Mountains hardwood forest,” Forest Ecology and Management, vol. 258, no. 3, pp. 233–242, 2009.
- T. S. Jung, I. D. Thompson, and R. D. Titman, “Roost site selection by forest-dwelling male Myotis in central Ontario, Canada,” Forest Ecology and Management, vol. 202, no. 1–3, pp. 325–335, 2004.
- M. A. Menzel, S. F. Owen, W. M. Ford et al., “Roost tree selection by northern long-eared bat (Myotis septentrionalis) maternity colonies in an industrial forest of the central Appalachian mountains,” Forest Ecology and Management, vol. 155, no. 1–3, pp. 107–114, 2002.
- R. W. Perry and R. E. Thill, “Roost selection by male and female northern long-eared bats in a pine-dominated landscape,” Forest Ecology and Management, vol. 247, no. 1–3, pp. 220–226, 2007.
- D. B. Sasse and P. J. Pekins, “Summer roosting ecology of northern long-eared bats (Myotis septentrionalis) in the White Mountain National Forest,” in Proceedings of the Bats and Forests Symposium, R. M. R. Barclay and R. M. Brigham, Eds., pp. 91–101, British Columbia Ministry of Forests, 1996.
- J. C. Timpone, J. G. Boyles, K. L. Murray, D. P. Aubrey, and L. W. Robbins, “Overlap in roosting habits of Indiana bats (Myotis sodalis) and northern bats (Myotis septentrionalis),” American Midland Naturalist, vol. 163, no. 1, pp. 115–123, 2010.
- M. J. Evelyn, D. A. Stiles, and R. A. Young, “Conservation of bats in suburban landscapes: roost selection by Myotis yumanensis in a residential area in California,” Biological Conservation, vol. 115, no. 3, pp. 463–473, 2004.
- V. J. Bakker and K. Hastings, “Den trees used by northern flying squirrels (Glaucomys sabrinus) in southeastern Alaska,” Canadian Journal of Zoology, vol. 80, no. 9, pp. 1623–1633, 2002.
- H. M. Hackett and J. F. Pagels, “Nest site characteristics of the endangered northern flying squirrel (Glaucomys sabrinus coloratus) in Southwest Virginia,” American Midland Naturalist, vol. 150, no. 2, pp. 321–331, 2003.
- L. McDonald, “Relationships between northern flying squirrels and stand age and structure in aspen mixedwood forests in Alberta,” in Relationships between Stand Age, Stand Structure, and Biodiversity in Aspen Mixedwood Forests in Alberta, J. B. Stelfox, Ed., pp. 227–231, Alberta Environmental Centre, Vegreville, Canada; Canadian Forest Service, Edmonton, Canada, 1995.
- P. D. Weigl, “Study of the northern flying squirrel, Glaucomys sabrinus, by temperature telemetry,” American Midland Naturalist, vol. 92, no. 2, pp. 482–486, 1974.
- M. J. Merrick, S. R. Bertelsen, and J. L. Koprowski, “Characteristics of mount graham red squirrel nest sites in a mixed conifer forest,” Journal of Wildlife Management, vol. 71, no. 6, pp. 1958–1963, 2007.
- E. L. Bull and T. W. Heater, “Resting and denning sites of American martens in Northeastern Oregon,” Northwest Science, vol. 74, no. 3, pp. 179–185, 2000.
- T. G. Chapin, D. M. Phillips, D. J. Harrison, et al., “Seasonal selection of habitat by resting marten in Maine,” in Martes: Taxonomy, Ecology, Techniques, and Management, G. Proulx, H. N. Bryant, and P. M. Woodard, Eds., pp. 166–181, Provincial Museum of Alberta, Edmonton, Canada, 1997.
- J. H. Gilbert, J. L. Wright, D. J. Lauten, et al., “Den and rest-site characteristics of American marten and fisher in northern Wisconsin,” in Martes: Taxonomy, Ecology, Techniques, and Management, G. Proulx, H. N. Bryant, and P. M. Woodard, Eds., pp. 135–1145, Provincial Museum of Alberta, Edmonton, Canada, 1997.
- T. N. Hauptman, Spatial and temporal distribution and feeding ecology of the pine marten [M.S. thesis], Idaho State University, Pocatello, Idaho, USA, 1979.
- G. R. Ryder, “Characteristics of three natal den sites of American marten in the lower mainland region of southwestern British Columbia,” Wildlife Afield, vol. 6, no. 1, pp. 32–35, 2009.
- K. M. Wynne and J. A. Sherburne, “Summer home range use by adult marten in northwestern Maine,” Canadian Journal of Zoology, vol. 62, no. 5, pp. 941–943, 1984.
- S. M. Arthur, W. B. Krohn, and J. R. Gilbert, “Habitat use and diet of fishers,” Journal of Wildlife Management, vol. 53, no. 3, pp. 680–688, 1989.
- A. K. Mazzoni, Habitat use by fishers (Martes pennanti) in the southern Sierra Nevada, California [M.S. thesis], California State University, Fresno, Calif, USA, 2002.
- T. F. Paragi, S. M. Arthur, and W. B. Krohn, “Importance of tree cavities as natal dens for fishers,” Northern Journal of Applied Forestry, vol. 13, no. 2, pp. 79–83, 1996.
- R. A. Powell and W. J. Zielinsky, “Fisher,” in The Scientific Basis For Conserving Forest Carnivores: American Marten, Fisher, Lynx and Wolverine in the Western United States, K. B. Aubry, S. W. Buskirk, L. J. Lyon, and W. J. Zielinski, Eds., pp. 38–73, USDA Forest Service General Technical Report RM-254, 1994.
- C. M. Raley, “Ecological characteristics of fishers (Martes pennanti) in the Southern Oregon Cascade Range, update: July 2006. Report,” in USDA Forest Service, pp. 1–31, Olympia Forestry Sciences Laboratory, Olympia, Wash, USA, 2006.
- R. D. Weir, Diet, spatial organization, and habitat relationships of fishers in south-central British Columbia [M.S. thesis], Simon Fraser University, Burnaby, Canada, 1995.
- R. D. Weir, Fisher Ecology in the Kiskatinaw Plateau Ecosection, Year-End Report, Ministry of Environment of British Columbia, Victoria, Canada, 2008.
- R. D. Weir, F. Corbould, and A. Harestad, “Effect of ambient temperature on the selection of rest structures by fishers,” in Martens and Fishers (Martes) in HumAn-Altered Environments: An International Perspective, D. J. Harrison, A. K. Fuller, and G. Proulx, Eds., pp. 187–197, Springer Science and Business Media, New York, NY, USA, 2004.
- J. J. Beecham, D. G. Reynolds, and M. G. Hornocker, “Black bear denning activities and den characteristics in west-central Idaho,” Bears: Their Biology and Management, vol. 5, pp. 79–86, 1983.
- E. L. Bull, J. J. Akenson, and M. G. Henjum, “Characteristics of black bear dens in trees and logs in northeastern Oregon,” Northwest Naturalist, vol. 81, no. 3, pp. 148–153, 2000.
- A. W. Erickson, B. M. Hanson, and J. J. Brueggeman, “Black bear denning study, Mitkof Island, Alaska,” Project Report FRI-UW-8214, School of Fisheries, University of Washington, Seattle, Wash, USA, 1982.
- T. K. Fuller and L. B. Keith, “Summer ranges, cover type use, and denning of black bears near Fort. McMurray, Alberta,” The Canadian Field-Naturalist, vol. 94, no. 1, pp. 80–83, 1980.
- K. G. Johnson and M. R. Pelton, “Selection and availability of dens for black bears in Tennessee,” Journal of Wildlife Management, vol. 45, no. 1, pp. 111–119, 1981.
- C. J. Jonkel and I. M. Cowan, “The black bear in the spruce-fir forest,” Wildlife Monographs, no. 27, pp. 1–55, 1971.
- G. B. Kolenosky and S. M. Strathearn, “Winter denning of black bears in east-central Ontario,” Bears: Their Biology and Management, vol. 7, pp. 305–316, 1987.
- D. A. Martorello and M. R. Pelton, “Microhabitat characteristics of American black bear nest dens,” Ursus, vol. 14, no. 1, pp. 21–26, 2003.
- Manning, Cooper and Associates, 2002 Black Bear Winter Den Inventory. TFL 37, Northern Vancouver Island, BC, Canadian Forest Products, Woss, Canada, 2003.
- M. K. Oli, H. A. Jacobson, and B. D. Leopold, “Denning ecology of black bears in the White River National Wildlife Refuge, Arkansas,” Journal of Wildlife Management, vol. 61, no. 3, pp. 700–706, 1997.
- C. W. Ryan and M. R. Vaughan, “Den characteristics of black bears in southwestern Virginia,” Southeastern Naturalist, vol. 3, no. 4, pp. 659–668, 2004.
- W. G. Wathen, K. G. Johnson, and M. R. Pelton, “Characteristics of black bear dens in the southern Appalachian region,” Bears: Their Biology and Management, vol. 6, pp. 119–127, 1986.
- T. H. White, J. L. Bowman, H. A. Jacobson, B. D. Leopold, and W. P. Smith, “Forest management and female black bear denning,” Journal of Wildlife Management, vol. 65, no. 1, pp. 34–40, 2001.
- F. L. Bunnell and I. Houde, “Down wood and biodiversity—implications to forest practices,” Environmental Reviews, vol. 8, pp. 397–421, 2010.
- D. Huber and H. U. Roth, “Denning of brown bears in Croatia,” Bears: Their Biology and Management, vol. 9, pp. 79–83, 1997.
- K. Elgmork, “Denning behaviour of a female brown bear, Ursus arctos (Linne, 1758), with three young,” Säugetierkundliche Mitteilungen, vol. 29, no. 3, pp. 59–66, 1981.
- S. P. Cline, The characteristics and dynamics of snags in Douglas-fir forests of the Oregon Coast Range [Ph.D. thesis], Oregon State University, Corvallis, Ore, USA, 1977.
- S. G. Nilsson, “The evolution of nest-site selection among hole-nesting birds: the importance of nest predation and competition.,” Ornis Scandinavica, vol. 15, no. 3, pp. 167–175, 1984.
- A. Nappi and P. Drapeau, “Reproductive success of the black-backed woodpecker (Picoides arcticus) in burned boreal forests: are burns source habitats?” Biological Conservation, vol. 142, no. 7, pp. 1381–1391, 2009.
- N. Nielsen-Pincus, Nest site selection, nest success, and density of selected cavity-nesting birds in northeastern Oregon with a method for improving accuracy of density estimates [M.S. thesis], University of Idaho, Moscow, Idaho, USA, 2005.
- W. F. Laudenslayer Jr., “Cavity-nesting bird use of snags in eastside pine forests of northeastern California,” in Proceedings of the Symposium on the Ecology and Management of Dead Wood in Western Forests, W. F. Laudenslayer Jr., P. J. Shea, B. E. Valentine, C. P. Weatherspoon, and T. E. Lisle, Eds., pp. 223–236, USDA Forest Service, General Technical Report PSW-GTR-181, 2002.
- S. T. Walter and C. C. Maguire, “Snags, cavity-nesting birds, and silvicultural treatments in western Oregon,” Journal of Wildlife Management, vol. 69, no. 4, pp. 1578–1591, 2005.
- R. J. Fisher and K. L. Wiebe, “Nest site attributes and temporal patterns of northern flicker nest loss: effects of predation and competition,” Oecologia, vol. 147, no. 4, pp. 744–753, 2006.
- K. J. Gutzwiller and S. H. Anderson, “Multiscale associations between cavity-nesting birds and features of Wyoming streamside woodlands,” Condor, vol. 89, no. 3, pp. 534–548, 1987.
- C. L. Hartwig, Effect of forest age, structural elements, and prey density on the relative abundance of Pileated Woodpecker (Dryocopus pileatus abieticola) on southeastern Vancouver Island [M.S. thesis], University of Victoria, Victoria, Canada, 1999.
- T. K. Mellen, E. C. Meslow, and R. W. Mannan, “Summertime home range and habitat use of pileated woodpeckers in western Oregon,” Journal of Wildlife Management, vol. 56, no. 1, pp. 96–103, 1992.
- R. G. Troetschler, “Acorn woodpecker breeding strategy as affected by starling nest-hole competition,” The Condor, vol. 78, no. 2, pp. 151–165, 1976.
- B. G. Hill and M. R. Lein, “Ecological relations of sympatric black-capped and mountain chickadees in southwestern Alberta,” The Condor, vol. 90, no. 4, pp. 875–884, 1988.
- S. M. Ramsay, K. Otter, and L. M. Ratcliffe, “Nest-site selection by female black-capped Chickadees: settlement based on conspecific attraction?” Auk, vol. 116, no. 3, pp. 604–617, 1999.
- A. E. Allin, “Nesting of the barred owl (Strix varia) in Ontario,” The Canadian Field-Naturalist, vol. 58, pp. 8–9, 1944.
- K. R. Bevis, “Primary excavators in grand fir forests of Washington's east Cascades and forestry on the Yakima Indian Nation, Washington,” in Proceedings of the Wildlife Tree/Stand-Level Biodiversity Workshop, P. Bradford, T. Manning, and B. I'Anson, Eds., pp. 77–86, BC Ministry of Environment, Lands, and Parks and Ministry of Forests, Victoria, Canada, 1996.
- D. J. Spiering and R. L. Knight, “Snag density and use by cavity-nesting birds in managed stands of the Black Hills National Forest,” Forest Ecology and Management, vol. 214, no. 1–3, pp. 40–52, 2005.
- A. B. Carey, M. M. Hardt, S. P. Horton, et al., “Spring bird communities in the Oregon Coast Range,” in Wildlife and Vegetation of Unmanaged Douglas-Fir Forests, L. F. Ruggiero, K. B. Aubry, A. B. Carey, and M. F. Huff, Eds., pp. 123–144, USDA Forest Service General Technical Report, PNW-GTR-285, 1991.
- R. W. Mannan, Use of snags by birds, Douglas-fir region, western Oregon [M.S. thesis], Oregon State University, Corvallis, Ore, USA, 1977.
- R. W. Mannan and E. C. Meslow, “Bird populations and vegetation characteristics in managed and old-growth forests, northeastern Oregon,” Journal of Wildlife Management, vol. 48, no. 4, pp. 1219–1238, 1984.
- J. E. Zarnowitz and D. A. Manuwal, “The effects of forest management on cavity-nesting birds in northwestern Washington,” Journal of Wildlife Management, vol. 49, no. 1, pp. 255–263, 1985.
- C. Steeger and H. Quesnel, “Impacts of partial cutting on old-growth forests in the Rocky Mountain trench: interim report,” Tech. Rep. 9, Enhanced Forest Management Pilot Project, Invermere, Canada, 1998.
- R. L. Hutto, “Toward meaningful snag-management guidelines for postfire salvage logging in North American conifer forests,” Conservation Biology, vol. 20, no. 4, pp. 984–993, 2006.
- E. L. Bull, C. G. Parks, and T. Torgerson, Trees and Logs Important to Wildlife in the Interior Columbia River Basin, USDA Forest Service General Technical Report PNW-GTR-391, 1997.
- W. C. McComb, S. A. Bonney, R. M. Sheffield, and N. D. Cost, “Snag resources in Florida—are they sufficient for average populations of primary cavity-nesters?” Wildlife Society Bulletin, vol. 14, no. 1, pp. 40–48, 1986.
- G. A. McPeek, W. C. McComb, J. J. Moriarty, et al., “Bark-foraging bird abundance unaffected by increased snag availability in a mixed mesophytic forest,” The Wilson Bulletin, vol. 99, no. 2, pp. 253–257, 1987.
- W. A. Nietro, V. W. Binkley, S. P. Cline, et al., “Snags (wildlife trees),” in Management of Wildlife and Fish Habitats in Forests of Western Oregon and Washington, E. R. Brown, Ed., pp. 129–169, USDA Forest Service Publication R6-F&WL-192-1985, 1985.
- F. L. Bunnell, T. Spribille, I. Houde, T. Goward, and C. Björk, “Lichens on down wood in logged and unlogged forest stands,” Canadian Journal of Forest Research, vol. 38, no. 5, pp. 1033–1041, 2008.
- F. R. Larson, Downed Woody Material in Southeast Alaska Forest Stands, USDA Forest Service Research Paper PNW-RP-452, 1992.
- P. Sollins, “Input and decay of coarse woody debris in coniferous stands in western Oregon and Washington,” Canadian Journal of Forest Research, vol. 12, no. 1, pp. 18–28, 1982.
- B. G. Marcot, J. L. Ohmann, K. L. Mellen-McLean, and K. L. Waddell, “Synthesis of regional wildlife and vegetation field studies to guide management of standing and down dead trees,” Forest Science, vol. 56, no. 4, pp. 391–404, 2010.
- D. J. Huggard, Synthesis of Studies of Forest Bird Responses to Partial-Retention Forest Harvesting, Pamphlet, Centre for Applied Conservation Research, University of British Columbia, Vancouver, Canada, 2006.
- F. L. Bunnell and B. G. Dunsworth, “Making adaptive management for biodiversity work—the example of Weyerhaeuser in coastal British Columbia,” Forestry Chronicle, vol. 80, no. 1, pp. 37–43, 2004.
- A. J. Huggett, “The concept and utility of “ecological thresholds” in biodiversity conservation,” Biological Conservation, vol. 124, no. 3, pp. 301–310, 2005.
- D. B. Lindenmayer and G. Luck, “Synthesis: thresholds in conservation and management,” Biological Conservation, vol. 124, no. 3, pp. 351–354, 2005.
- J. S. Guénette and M. A. Villard, “Thresholds in forest bird response to habitat alteration as quantitative targets for conservation,” Conservation Biology, vol. 19, no. 4, pp. 1168–1180, 2005.
- D. Huggard, “Forest birds and retention levels,” BC Journal of Ecosystems and Management, vol. 8, no. 3, pp. 120–124, 2007.
- F. L. Bunnell, M. Boyland, and E. Wind, “How should we spatially distribute dead and dying wood?” in Proceedings of the Symposium on the Ecology and Management of Dead Wood in Western Forests, W. F. Laudenslayer Jr., P. J. Shea, B. E. Valentine, C. P. Weatherspoon, and T. E. Lisle, Eds., pp. 739–752, USDA Forest Service, General Technical Report PSW-GTR-181, 2002.
- B. G. Marcot, “Snag use by birds in Douglas-fir clearcuts,” in Proceedings of the Symposium in Snag Habitat Management, J. W. Davis, G. A. Goodwin, and R. A. Ockenfels, Eds., pp. 134–139, USDA Forest Service General Technical Report RM-99, 1983.
- L. J. Bate, E. O. Garton, and M. J. Wisdom, Estimating Snag and Large Tree Densities and Distributions on a Landscape for Wildlife Management, USDA Forest Service General Technical Report PNW-GTR-425, Portland, Ore, USA, 1999.
- D. R. Petit, K. E. Petit, T. C. Grubb Jr., et al., “Habitat and snag selection by woodpeckers in a clear-cut: an analysis using artificial snags,” The Wilson Bulletin, vol. 97, no. 4, pp. 525–533, 1985.
- W. Walankiewicz, “Do secondary cavity-nesting birds suffer more from competition for cavities or from predation in a primeval deciduous forest,” Natural Areas Journal, vol. 11, no. 4, pp. 203–212, 1991.
- C. J. E. Welsh and D. E. Capen, “Availability of nesting sites as a limit to woodpecker populations,” Forest Ecology and Management, vol. 48, no. 1-2, pp. 31–41, 1992.
- J. G. Dickson, R. N. Conner, and J. H. Williamson, “Snag retention increases bird use of clear-cut,” Journal of Wildlife Management, vol. 47, no. 3, pp. 799–804, 1983.
- V. E. Scott, “Bird responses to snag removal in ponderosa pine,” Journal of Forestry, vol. 77, no. 1, pp. 26–28, 1979.
- J. L. Ohmann, W. C. McComb, and A. A. Zumrawi, “Snag abundance for primary cavity-nesting birds on nonfederal forest lands in Oregon and Washington,” Wildlife Society Bulletin, vol. 22, no. 4, pp. 607–619, 1994.
- F. L. Bunnell and G. B. Dunsworth, Eds., Forestry and Biodiversity. Learning How to Sustain Biodiversity in Managed Forests, University of British Columbia Press, Vancouver, Canada, 2009.
- B. Söderström, “Effects of different levels of green- and dead-tree retention on hemi-boreal forest bird communities in Sweden,” Forest Ecology and Management, vol. 257, no. 1, pp. 215–222, 2009.
- K. Mellen, B. G. Marcot, J. L. Ohmann, et al., “DecAID: a decaying wood advisory model for Oregon and Washington,” in Proceedings of the Symposium on the Ecology and Management of Dead Wood in Western Forests, W. F. Laudenslayer Jr., P. J. Shea, B. E. Valentine, C. P. Weatherspoon, and T. E. Lisle, Eds., pp. 527–533, USDA Forest Service, General Technical Report PSW-GTR-181, 2002.
- A. J. Erskine, Birds in Boreal Canada: Communities, Densities and Adaptations, Canadian Wildlife Service Report Series 41, Ottawa, Canada, 1977.
- M. E. Harmon, J. F. Franklin, F. J. Swanson et al., “Ecology of coarse woody debris in temperate ecosystems,” Advances in Ecological Research, vol. 15, pp. 133–302, 1986.
- M. C. Vanderwel, J. R. Malcolm, and S. M. Smith, “Long-term snag and downed woody debris dynamics under periodic surface fire, fire suppression, and shelterwood management,” Canadian Journal of Forest Research, vol. 39, no. 9, pp. 1709–1721, 2009.
- R. L. L. Graham, Biomass dynamics of dead Douglas-fir and western hemlock boles in mid- elevation forests of the Cascade Range [Ph.D. thesis], Oregon State University, Corvallis, Ore, USA, 1981.
- USDA Forest Service, Ecological Characteristics of Fishers (Martes Pennanti) in the Southern Oregon Cascade Range, USDA Forest Service, Pacific Northwest Research Station, Olympia, Wash, USA, 2006.
- C. S. Binkley, “Preserving nature through intensive plantation forestry: the case for forestland allocation with illustrations from British Columbia,” Forestry Chronicle, vol. 73, no. 5, pp. 553–559, 1997.
- F. L. Bunnell, R. W. Wells, J. D. Nelson, et al., “Effects of harvest policy on landscape pattern, timber supply and vertebrates in an East Kootenay watershed,” in Forest Fragmentation: Wildlife and Management Implications, J. A. Rochelle, L. A. Lehmann, and J. Wisniewski, Eds., pp. 271–293, Brill, Leiden, The Netherlands, 1999.
- E. C. Lofroth, Scale dependent analyses of habitat selection by marten in the sub-boreal spruce biogeoclimatic zone, British Columbia [M.S. thesis], Simon Fraser University, Burnaby, Canada, 1993.
- K. D. Coates, “Windthrow damage 2 years after partial cutting at the Date Creek silvicultural systems study in the interior Cedar-Hemlock forests of northwestern British Columbia,” Canadian Journal of Forest Research, vol. 27, no. 10, pp. 1695–1701, 1997.
- J. F. Franklin, D. R. Berg, D. A. Thornburgh, et al., “Alternative silvicultural approaches to timber harvesting: variable retention harvest systems,” in Creating a Forestry for the 21st Century: The Science of Ecosystem Management, K. A. Kohm and J. F. Franklin, Eds., pp. 111–139, Island Press, Washington, DC, USA, 1997.
- R. M. S. Vega, Bird communities in managed conifer stands in the Oregon Cascades: habitat associations and nest predation [M.S. thesis], Oregon State University, Corvallis, Ore, USA, 1993.
- F. L. Bunnell, L. L. Kremsater, and I. Houde, “Mountain pine beetle: a synthesis of the ecological .consequences of large-scale disturbances on sustainable forest management, with emphasis on biodiversity,” Information Report BC-X-426, Natural Resources Canada, Canadian Forest Service, Pacific Forestry Centre, Victoria, Canada, 2011.
- J. D. McIver and L. Starr, “A literature review on the environmental effects of postfire logging,” Western Journal of Applied Forestry, vol. 16, no. 4, pp. 159–168, 2001.