Parental care trade-offs in the inter-brood phase in Barn Swallows Hirundo rustica
In seasonal environments with limited time and energy resources, double-brooded birds face trade-offs in the timing of their two reproductive attempts and in the effort allocated to the first and the second broods. In the Barn Swallow Hirundo rustica a long care period for the first brood enhances the survival of first-brood chicks, but also delays the start of the second brood, which in turn reduces the survival prospects of second-brood chicks. Probably as a response to this trade-off, double-brooded Barn Swallows reduce the period of post-fledging care for first-brood fledglings. By radiotracking whole families, we investigated the determinants of this behaviour and its consequences for the survival of the first-brood fledglings. The end of the females’ investment in post-fledging care of the first brood was related to the beginning of egg synthesis for the second clutch. With the start of egg synthesis, females significantly reduced provisioning rates to the first-brood fledglings to less than one-fifth of the previous rates, while the proportion of time they spent foraging remained high. Assuming that the females’ foraging success was constant, we conclude that their energy income was allocated to egg production rather than fledgling provision. Males did not compensate for the females’ reduced feeding rates. Thus the start of egg production for the second clutch had a marked effect on the quantity of food received by first-brood fledglings. In parallel with the changes in parental behaviour and provisioning rates, we observed a marked drop in the daily survival rate of first-brood chicks. These results support the hypothesis that females face a strong trade-off in the allocation of energy to subsequent broods. Energy allocation to a second clutch involves a cost in terms of reduced provisioning, and as a result the survival of first-brood chicks is compromised. This is probably outweighed by the improved success of an early second brood.