Clutch size of the Pied Flycatcher Ficedula hypoleuca-an experiment

Abstract

We manipulated the sizes of Pied Flycatcher broods, to study whether females layclutches that maximize their life-time reproductive success, and whether clutch size is mainly limitedby the capacity of the parents to feed theoffspring. Soon after hatching brood sizes were increased or reduced by one or two nestlings. The number of young leaving the nest was higher in broods with extra young, but the fledging weight was lower. As Pied Flycatchers in Scandinavia have high natal dispersal, we could not count the number of offspring surviving to adulthood. In the sibling species, the Collared Flycatcher Ficedula albicollis, on the island of Gotland the young return to breedin their natalareas and in this species the likelihood of survival was observed to increase with the weight at fledging. When we used this relationship to estimate the likelihood of survival forthe Pied Flycatchers, the broods with extra young were not found to be more productive than broods of the original size, which suggests that brood size is mainly limited by thecapacity of the parents to feed the young. The manipulation did not influence the likelihood of survival of the parent birds, which suggests that trade-offbetween adult survival andreproductive effort is not important