Despite recent progress in avian demography, much still needs to be learned about patterns of survival and dispersal across different ages and sexes of birds. This is expressly true for Brown-headed Cowbirds (Molothrus ater) in light of their vagility and importance in the life history of many other bird species. We used multistate mark—recapture models to estimate annual survival and dispersal rates of Brown-headed Cowbirds across dialect subpopulations in the Sierra Nevada of California across the 1982–1988 breeding seasons. We estimated rates for male and female hatch-year (HY) and after-hatch-year (AHY) birds and found no evidence of differences in capture probabilities across sex and age states. To account for high apparent nonresidency, adult survival was estimated separately for the year following initial capture and for subsequent years. The latter estimates are near the midpoint of those from other studies (AHY females: 56.1%; after-second-year [ASY] males: 63.4%; second-year [SY] males: 32.9%). Survival estimates for HY birds are high for passerines (females: 24.6%; males: 25.3%), although they are largely consistent with estimates from long-term mark—recapture studies in other species. We found no evidence of sex-biased dispersal, although rates for HY birds (about 14–15%) were about 3× higher than those for adults (about 4–5%). Our study provides a more detailed understanding of the demography and dispersal of cowbirds, offering a platform for future research on the social behavior and ecology of this species and on possible strategies for its management.
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Vol. 129 • No. 2