FEEDING TIME AND BROOD REARING CAPACITY IN THE PIED FLYCATCHER FICEDULA-HYPOLEUCA

Lack (1947) suggested that one factor affecting clutch size in birds i s the latitude at which the clutch is laid. He proposed that this geog raphical variation is partly influenced by latitudinal differences in day length. We studied the effect of reduced daily feeding time (day l ength) on the breeding success of the Pied Flycatcher, Ficedula hypole uca, and whether parents were able to compensate for the lost feeding time. We kept both adults captive for 2-5 hours each day when the nest lings were from 8 to 13 days old and recorded the effects on the devel opment of the offspring. The nestlings with reduced feeding time were lighter when fledging and had a lower expected survival rate than thos e of the control nests. The results indicate that the adults were unab le to compensate for the lost feeding time, since the feeding rate did not differ between the experimental and control nests. Since some bir d species feed the oldest nestling of the brood more frequently than t he younger ones when they cannot find enough food for all the nestling s, we calculated the coefficient of variation (CV) for nestling weight s for each brood. The results from this analysis suggested that all th e nestlings within each brood developed in an almost identical manner. Our results suggest that day length may be critical for the ability o f the parents to feed their nestlings, supporting Lack's findings that day length may influence geographical variation in brood size.