HOST RESPONSES TO COWBIRDS NEAR THE NEST - CUES FOR RECOGNITION

Two cues (bill shape and vocalizations) that yellow warblers, Dendroic a petechia, may use to recognize brown-headed cowbirds, Molothrus ater , were examined experimentally. Female yellow warblers responded more intensely to a control cowbird than one with a longer, thinner bill, w hich suggests that bill shape was an important cue in recognition. Res ponses involved close approaches, 'seet' alarm calls and sitting in th e nest. Female yellow warblers distinguished between vocalizations of male and female cowbirds and those of a song sparrow, Melospiza melodi a (control). They responded most intensely to female cowbird 'chatter calls' and least intensely to the sparrow song; responses to male cowb ird 'perched song' were intermediate in intensity. The results suggest that female yellow warblers recognize cowbirds on the basis of bill s hape and vocalizations. Furthermore, these results, coupled with previ ous findings, suggest that female yellow warblers distinguish between male and female cowbirds by vocalizations but not appearance. (C) 1997 The Association for the Study of Animal Behaviour.