Postfledging parental effort in barn swallows: evidence for a trade-off in the allocation of time between broods

In altricial birds, parental care after fledging should be terminated as soon as the benefits from deserting the young outweigh those from continuing parental care. In seasonal environments, multiple breeders may trade off fitness benefits of the time invested in the care of fledglings of one brood against the benefits of an advanced start of the subsequent clutch. Using radiotelemetry of 494 fledglings in 114 families (of 88 pairs) of the double-brooded barn swallow, Hirundo rustica, we found that, irrespective of the offspring quality, the duration of postfledging parental care was shorter for first broods than for single and second broods. A shorter duration of postfledging care was associated with a steeper decline in feeding rates after fledging. Postfledging parental care also depended on the pair's breeding season length and on the phenotypic quality of the female parent. We suggest that pairs that start other reproductive bouts in the same season and pairs with high chances of survival to subsequent years reduce parental effort in the current brood to maximize lifetime reproductive output. The results support the hypothesis that multibrooded birds adjust the duration of postfledging care in relation to future reproductive attempts because of an intraseasonal trade-off in the allocation of time between successive broods.