Possible roles for corticosterone and critical size in the fledging of nestling pied flycatchers

Our study was designed to see whether corticosterone (B) rises abruptly in the blood of nestling pied flycatchers (Ficedula hypoleuca) at the time the y fledge, as reported recently for kestrels, and if so, why. We measured th e growth and blood levels of B and selected nutrients of nestlings in brood s of five, seven, and nine chicks during 1998 and 1999. In half of the broo ds, we clipped selected wing and tail feathers of both parents with the int ention of making it more difficult for them to provide their chicks with fo od. We collected blood samples when the chicks were six to 10 d old (period of rapid growth) and 15 d of age or older (0-5 d before fledging). B incre ased substantially several days before the chicks left the nest and then de clined somewhat. We found no differences in rates of growth or blood levels of B, nutrients, and hematocrit as a function of either brood size or pare ntal handicapping. Nestlings within a day of fledging appear to have been f ood deprived in 1998; their glucose was significantly reduced, and B, free fatty acids, and glycerol were significantly elevated compared to levels in chicks 1-4 d younger. Such changes did not occur in 1999. Blood levels of B were significantly correlated with brood size near the day of fledging, b ut not earlier, in both years of the study. It was possible to predict the day on which chicks would leave the nest, using their wing length when 12 d old. These results suggest that high blood levels of B associated with foo d restriction and sibling competition induce chicks to fledge, provided the y have reached a critical size, and that the importance of fasting, sibling competition, and B may vary from year to year.