Mass Loss in Breeding Blue Tits: The Role of Energetic Stress

Abstract

1. The hypothesis that mass reduction in breeding passerines results from energetic stress was evaluated using data on body mass changes in female blue tits Parus caeruleus. 2. In accordance with both the adaptive adjustment and the physiological stress hypotheses, females with experimentally enlarged broods lost more mass than females rearing reduced or control broods. However, the ability to allocate energy to self-maintenance (as measured by the regrowth rate of a tail feather removed during incubation) was negatively correlated with the amount of lost body mass. 3. In one of the study years, loss of body mass was more pronounced among small females, suggesting that larger females are better able to cope with poor food conditions during breeding. 4. In a poor-weather year, 30% of the females deserted their clutches, compared with 8% in a good year. Females that deserted their clutches before hatching were significantly lighter during incubation than non-deserters, indicating that good body condition is important for successful reproduction. 5. In one year young females lost more mass than older females and therefore the ability to maintain adequate body condition in the face of energetic stress appears to be age-dependent. 6. Taken together, these results suggest that mass loss in breeding blue tits is, to some degree, attributable to energetic stress, although we have not ruled out the possibility that flight cost reductions may help explain the phenomenon.