Host responses to cowbirds near the nest: cues for recognition

Two cues (bill shape and vocalizations) that yellow warblers, Dendroica petechiamay use to recognize brown-headed cowbirds, Molothrus aterwere examined experimentally. Female yellow warblers responded more intensely to a control cowbird than one with a longer, thinner bill, which suggests that bill shape was an important cue in recognition. Responses involved close approaches, 'seet' alarm calls and sitting in the nest. Female yellow warblers distinguished between vocalizations of male and female cowbirds and those of a song sparrow, Melospiza melodia (control). They responded most intensely to female cowbird 'chatter calls' and least intensely to the sparrow song; responses to male cowbird 'perched song' were intermediate in intensity. The results suggest that female yellow warblers recognize cowbirds on the basis of bill shape and vocalizations. Furthermore, these results, coupled with previous findings, suggest that female yellow warblers distinguish between male and female cowbirds by vocalizations but not appearance.