Development and stability of single-parent family units in the song sparrow

Allocation of parental feedings to newly fledged young was observed in 34 song sparrow broods on the day of leaving the nest (day 0) and in five more broods the subsequent day (day 1). In the one case in which leaving the nest was observed, feedings were immediately divided between parents so that each young was fed by only one parent. Four-fifths of all young were fed by only a single parent on day 0 or day 1: the remaining fifth were fed by both parents during observation periods of about 90 min. Twenty-nine of the same broods were observed again during days 4–8, when only 1 of 82 young was fed by both parents. Slightly more young were fed by males than on days 0–1. The tendency for males to care for a higher proportion of older young occurred whether or not the pair began a further brood. Song sparrow fledglings gave individually distinct food begging calls. These may have assisted the formation or maintenance of specific parent–young feeding units. Brown-headed cowbird young in the brood were treated like song sparrows, except that they were fed more frequently.