THE RELATIONSHIP BETWEEN THE ENERGY-EXPENDITURE DURING INCUBATION AND CLUTCH SIZE IN THE PIED FLYCATCHER FICEDULA-HYPOLEUCA

In a population of Pied Flycatchers breeding in nestboxes in a montane pine forest in central Spain, we performed a clutch manipulation experiment by adding or removing one egg in certain nests, while keeping the original number in other nests to serve as controls. No significant effect of clutch manipulations was detected with respect to female mass/condition at the beginning of incubation and mass/condition changes. DEE (daily energy expenditure) of incubating females, measured with doubly-labelled water, was lower for reduced than for control and enlarged clutches. DEE showed a nonlinear increasing relationship with clutch size. For nine females laying clutches of 5 there was a significant positive effect of increased clutch size on DEE, the number of incubated eggs explaining 77% of the variation in DEE. For females laying 6 eggs there was no apparent trend in DEE with clutch size, suggesting that females laying large clutches do not increase DEE further when incubating enlarged clutches.