Division of labour by song sparrows feeding fledged young

Abstract

Parental feeding of nestlings and fledglings is described in an individually marked population of song sparrows. Thirty-five broods of nestlings were observed in 1976 and 68 broods of fledglings in 1977. On leaving the nest, individual young dispersed to separate locations in the breeding territory where, at 13 days of age, a single parent fed each young. In broods with even numbers of young, male and female parents divided the young equally in most cases. With odd numbers of young, either male or female could feed the extra young, but females did so more often. Older young were more mobile and harder to observe, but there was some evidence that family division persisted. Male parents played a larger role in feeding older young than younger ones. Feeding rates of 13-day-old young were 44% higher than those of 7- to 8-day-old nestlings, suggesting that energy demands of young and investment of parents are maximal after young leave the nest. I suggest that division of labour helps parents to locate young and regulate their parental feeding efficiently.