Breeding time and sex-specific health status in the barn swallow (Hirundo rustica)

Abstract

Health status has been hypothesized to be an important factor associated with individual reproductive performance in birds. Sex-specific health status of early- and late-breeding barn swallows (Hirundo rustica) was studied during the prelaying and brood-rearing periods to test the prediction that early-arriving birds are in better health status than late-arriving individuals. During prelaying, early breeders (of both sexes) had lower leukocyte counts than late breeders, and males had fewer lymphocytes, heterophils, and total white blood cells than females. Sex differences in health status at arrival disappeared during brood rearing, owing to an increase in leukocytes of early-breeding males and a decrease in leukocytes of late-breeding females. Late-breeding males had a higher plasma lutein concentration than early breeders during brood rearing, and the lutein concentration increased significantly for both sexes between prelaying and brood rearing. T-cell immune response of late-breeding females was stronger than for early breeders. These results are consistent with the prediction that health status of breeding barn swallows affects individual performance in terms of breeding time. Individuals differing in quality experience different costs and benefits of early breeding, and these costs and benefits are sex specific.