Abstract
An unbalanced sex ratio with a predominance of males in the Brown-headed Cowbird (Molothrus ater) has been documented by Friedmann (1929), Mcllhenney (1940), and others. These workers did not investigate sexes in different age groups. Recently Darley (1971) found the sex ratio to be equal in second-year birds. I therefore investigated the sex ratio of more than 7,000 immature (first-year) cowbirds captured in westcentral Kansas. A protracted postjuvenal molt often complicates sexing of immatures by plumage during late summer. Wing length criteria proposed by Wood (1969) were not accurate for this study, so I propose alternative criteria.
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