VARIATION IN THE STRUCTURE OF FEMALE BROWN-HEADED COWBIRD VOCALIZATIONS AND ITS RELATION TO VOCAL FUNCTION AND DEVELOPMENT

Female Brown-headed Cowbirds (Molothrus ater) are unusual among temper ate zone songbirds in that they possess a distinctive and loud call, k nown as ''chatter'' and consisting of a series of brief chevron-shaped syllables. Unlike the high-amplitude vocalizations commonly given by males, which show extreme macrogeographic and microgeographic (i.e., d ialect) variation, chatter shows no apparent qualitative variation thr oughout North America. Quantitative variation in frequency and tempora l parameters was assessed by measuring 40 variables on chatter calls g iven by 45 females from the three recognized cowbird subspecies. Univa riate and multivariate (principle components) analyses showed only min or macrogeographic differences among the subspecies and gave no indica tion that local microgeographic variation or dialects exist. Despite t he near lack of spatial variation, analysis of variance demonstrated t hat nearly every variable shows significant individual variation withi n each subspecies. This is probably sufficient to allow cowbirds to re cognize individual females. Chatter is used in diverse contexts and in communication with both males and other females. Its quantitative fea tures may vary according to context, but this was not investigated bec ause all calls in this study were given in the same context.