Feeding time and brood rearing capacity in the Pied Flycatcher Ficedula hypoleuca

Abstrakti

Lack (1947) suggested that one factor affecting clutch size in birds is the latitude at which the clutch is laid . He proposed that this geographical variation is partly influenced by latitudinal differences in day length. We studied the effect of reduced daily feeding time (day length) on the breeding success of the Pied Flycatcher, Ficedula hypoleuca, 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 nestlings were from 8 to 13 days old and recorded the effects on the development of the offspring. The nestlings with reduced feeding time were lighter when fledging and had a lower expected survival rate than those of the control nests. The results indicate that the adults were unable to compensate for the lost feeding time, since the feeding rate did not differ between the experimental and control nests. Since some bird species feed the oldest nestling of the brood more frequently than the younger ones when they cannot find enough food for all the nestlings, we calculated the coefficient of variation (CV) for nestling weights for each brood. The results from this analysis suggested that all the nestlings within each brood developed in an almost identical manner. Our results suggest that day length may be critical for the ability of the parents to feed their nestlings, supporting Lack's findings that day length may influence geographical variation in brood size.