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 temperate zone songbirds in that they possess a distinctive and loud call, known 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., dialect) variation, chatter shows no apparent qualitative variation throughout North America. Quantitative variation in frequency and temporal parameters was assessed by measuring 40 variables on chatter calls given by 45 females from the three recognized cowbird subspecies. Univariate and multivariate (principle components) analyses showed only minor macrogeographic differences among the subspecies and gave no indication that local microgeographic variation or dialects exist. Despite the near lack of spatial variation, analysis of variance demonstrated that nearly every variable shows significant individual variation within each subspecies. This is probably sufficient to allow cowbirds to recognize individual females. Chatter is used in diverse contexts and in communication with both males and other females. Its quantitative features may vary according to context, but this was not investigated because all calls in this study were given in the same context.