We live-trapped and banded Brown-headed Cowbirds (Molothrus ater, n = 1,722 individuals) to evaluate sex ratios and survival probabilities during the breeding season at the San Juan Basin Research Center, La Plata County, Colorado, from 1992 to 1999. For adult Brown-headed Cowbirds, sex ratios varied significantly among years, and sex ratios were always male-biased within a year. In hatch-year (HY) cowbirds, and within years, sex ratios were similarly male-biased, but there was no difference among years in HY sex ratios. Using the program MARK, for adult females, second-year (SY) males, and after-second-year (ASY) males, the most parsimonious model, suggested that some variation occurred among these groups in individual survival probability (φ;) to the next year after the initial year of capture and in all following years. However, this model indicated no differences among these groups to best describe the probability of recapturing an individual (P). For HY individuals, analyses in MARK indicated that the most parsimonious model, did not include any differences in survival probability between the sexes, and survival probability was constant for the cohorts and periods examined. We also found some evidence that the residency status of adult Brown-headed Cowbirds potentially affected their frequency of recapture in the following year. In adults, the male-biased sex ratios that we observed were very likely the result of decreased survival probability of adult females compared with both SY and ASY males. Additionally, the male-biased sex ratios of adults may have already begun to develop in HY individuals.
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Vol. 126 • No. 2