Can changes in provisioning by parent birds account for seasonally declining patterns of offspring recruitment?

Declining reproductive success among individuals that breed later in the season occurs in numerous taxa and is particularly well-documented in birds. Principal ideas advanced to explain this pattern, the date and parental quality hypotheses, consider the ultimate causes of this phenomenon and have received much attention; however, proximate mechanisms have not been clearly elucidated. Parental provisioning could mediate a seasonal decline in nestling fitness. We delayed hatch dates and manipulated brood sizes of Tree Swallows (Tachycineta bicolor (Vieillot, 1808)) to assess the ability of parents to compensate for deteriorating environmental conditions and increased demands of more chicks. We measured provisioning rates using audio recordings of nestlings begging. Brood size was the best predictor of provisioning frequency, with parents feeding larger broods more frequently than smaller ones. Delayed hatching did not reduce provisioning rate despite declining food abundance. Date and food abundance were unrelated to provisioning rate, suggesting no seasonal change in the quantity of food nestlings receive. However, provisioning frequency was informative about life-history strategies of Tree Swallows, showing that late breeders incurred the costs of deteriorating environmental conditions rather than passing these costs on to their offspring.