Early developmental conditions affect stress response in juvenile but not in adult house sparrows (Passer domesticus).

The short- and long-term consequences of developmental conditions on fitness have received growing attention because the environmental conditions during early life may influence growth, condition at independence, recruitment, reproductive success or survival. We tested here, in a natural house sparrow population, if early conditions during nestling stage affected the stress response of the birds (i) shortly after fledging and (ii) next year, during their first breeding. We experimentally manipulated brood size to mimic different rearing conditions, creating reduced (-2 chicks) and enlarged broods (+2 chicks), while in a third group brood size was not manipulated. Nestling nutrition state decreased with post-manipulation brood sizes as indicated by lower body mass. Fledglings with higher body mass at the age of ten days showed lower stress response than birds that were leaner at the same age. Fledglings reared in large broods showed a higher response to stress protocol than chicks from small broods, and this effect was in significant interaction with the age of fledglings at capture. This interaction indicates that the effects of the brood size became gradually smaller as the fledglings grew older and were further from their nestling period. The effects of early conditions vanished by the next year: the stress response of adult first time breeders was unrelated to the brood size they fledged from. These results suggest that stress response may reflect the actual state of an individual, rather than its developmental history.