EXPERIMENTALLY-INDUCED CLUTCH SIZE ENLARGEMENTS AFFECT REPRODUCTIVE SUCCESS IN THE PIED-FLYCATCHER

We performed a food provisioning experiment in a population of Pied Fl ycatchers Ficedula hypoleuca breeding at high altitude in central Spai n to test if food availability before and during laying determines clu tch size. Food was provided to one of two pairs with the same date of initiation of nest-building (15 dyads of subsequently reproducing pair s were thus created). Food provisioning began on the day of initiation of nest-building and ended on the day after the last egg was laid. Al though laying date was unaffected by the experiment, clutch size in th e experimental treatment was significantly larger. This result could i ndicate that food availability at laying (1) proximately constrained c lutch size or (2) that females evaluated future conditions for incubat ing eggs and feeding nestlings based on food availability at laying. R eproductive success (proportion of eggs that resulted in fledged young ) was significantly reduced in the experimental treatment. This effect suggest that supplemented females were tricked by the experiment into laying more eggs than the number of eggs they were able to incubate w ith success and the number of nestlings they were able to feed, a sour ce of error in clutch size adjustment which could be common in non-exp erimental situations.