Demography of Brown-headed Cowbirds at Delta Marsh, Manitoba

Available estimates of demographic parameters for Brown-headed Cowbirds (Mo lothrus ater) vary geographically. However, few estimates are based on long -term studies of marked individuals. We conducted a mark-recapture study on the population of cowbirds at Delta Marsh, Manitoba during the 1993-1998 b reeding seasons. We estimated annual survival, breeding site fidelity, and sex ratio, and compared those parameter estimates to other populations of B rown-headed Cowbirds. The Delta Marsh population had higher adult survival (male 90.1%; female 69.6%) and breeding site fidelity (males 66.9%, female 59.5%) than reported for other populations, and the sex ratio was significa ntly different from unity (1.9 males: 1 female). We suggest that difference s in survival and breeding-site fidelity between the Delta Marsh population and others may be due to differences in methods used to calculate paramete r estimates. In contrast, variation in sex ratios is likely real and due to differences in the local ecological conditions. In our population, high su rvivorship and breeding-site fidelity may lead to low recruitment of new bi rds into the resident population and intense competition for Limited breedi ng opportunities. The highly male biased sex ratio may result in strong sex ual-selection pressure on males competing for the limited breeding opportun ities. Those circumstances have implications for the social behavior and ma ting system of cowbirds.