Are fat reserves in migratory birds affected by condition in early life?

Abstract

Effects of environmental conditions experienced during early life, as indicated by nestling body condition and hatching date, on migratory body condition and fat reserves were studied using data on nestling Blue Tits Parus caeruleus recaptured during their first autumn migration. Body condition index and fat score recorded during the migration were strongly inter-correlated, indicating that much of the variation in body condition index is due to differences in amount of stored fat. Nestling body condition was found to be a good predictor of amount of subcutaneous fat during migration (four months later) for males, but less so for females. However, the amount of fat during migration was negatively correlated with hatching date and positively correlated with recapture date in females, while male fat reserves were independent of both hatching and recapture dates. These results suggest that differences in body condition established during early life may be maintained over considerable periods of time, and that the time window for selective events leading to positive covariance between nestling body condition and fitness can be much wider than previously appreciated. In particular, through its effects on the amount of migratory fat, nestling condition or some trait closely correlated with it, may affect the young bird's probability of survival during the energetically demanding migration. Based on the findings of this and related studies, we outline some general mechanistic models which may account for the association between early life condition and adult performance.