Hooded warblers (Wilsonia citrina) use a rich variety of call notes to defend nonbreeding territories. In the upper row, the left hand bark (a) shows reverberation. The upper central figure (b) shows the reverse chevron (down then up), a species-specific characteristic of barks given spontaneously in territorial maintenance. This is followed by a series of rapid call notes (chippity chups; (c)), highly variable in structure, used by aggressive individuals before attacking specific rivals. The four lower call notes (d) are from breeding individuals. Barks are used in defense of nestlings, and the high-pitch chevron note on far right shows and elevated frequency typically given by adults when a predator is near the nest.