THE RELATIONSHIP BETWEEN THE ENERGY-EXPENDITURE DURING INCUBATION ANDCLUTCH 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 expe riment by adding or removing one egg in certain nests, while keeping t he original number in other nests to serve as controls. No significant effect of clutch manipulations was detected with respect to female ma ss/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 enla rged clutches. DEE showed a nonlinear increasing relationship with clu tch size. For nine females laying clutches of 5 there was a significan t positive effect of increased clutch size on DEE, the number of incub ated 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 in cubating enlarged clutches.