Figure 3: Top left: in the hybrid zone between pied flycatcher (Ficedula hypoleuca) and collared flycatcher (Ficedula albicollis) hybrids showed intermediate prevalence of Haemoproteus blood parasites compared to the parental species, and also intermediate immune responses to infection [74] (photo shows a male hybrid F. hypoleuca × F. albicollis, courtesy of Miroslav Král). Bottom left: haematozoan blood parasites can be agents of severe selection in birds (photo shows Haemoproteus multipigmentatus infecting red blood cells of endemic Galápagos doves [84], courtesy of Gediminas Valkiunas). Right panels: in the mountain white-crowned sparrows (Zonotrichia leucophrys oriantha) heterozygote advantage in terms of reduced parasite load may counteract elevated parasitism of immigrants, who are likely to produce the most heterozygous offspring [75] (photo courtesy of Bob Steele). Figure reproduced with permission of the Royal Society of London from MacDougall-Shackleton et al. [75].