Determination of clutch size in Treecreepers Certhia familiaris under food and time constraints

Abstract

Brood size of Eurasian Treecreepers Certhia familiaris was manipulated in three years that differed in food abundance in order to detect factors that limit clutch size. The Tree-creeper parents were able to successfully raise enlarged broods only in a year with good food availability, but even then, the nestlings were of lower body weight than in reduced broods. Among the enlarged broods, the lowest breeding success was observed in a year with the lowest food abundance. Treecreepers with enlarged broods depleted their food supply in a year with scarce food, but not when food was abundant, and changes in foraging behaviour were observed only when food was scarce. Females and males did not increase their provisioning rates with brood size, but the responses of the sexes differed between years. Our results support Lack's hypothesis of clutch-size regulation: when food was scarce, Treecreepers laid the maximal number of eggs that they were able to raise successfully to fledging. However, trade-offs between current and future reproduction may also be important in clutch size determination of this species. Food abundance and limited foraging time seem to play major roles in limiting the clutch size of Treecreepers, but the importance of these factors differs among years.