MASS-LOSS IN BREEDING BLUE TITS - THE ROLE OF ENERGETIC STRESS

1. The hypothesis that mass reduction in breeding passerines results f rom energetic stress was evaluated using data on body mass changes in female blue tits Parus caeruleus. 2. In accordance with both the adapt ive adjustment and the physiological stress hypotheses, females with e xperimentally enlarged broods lost more mass than females rearing redu ced or control broods. However, the ability to allocate energy to self -maintenance (as measured by the regrowth rate of a tail feather remov ed during incubation) was negatively correlated with the amount of los t body mass. 3. In one of the study years, loss of body mass was more pronounced among small females, suggesting that larger females are bet ter able to cope with poor food conditions during breeding. 4. In a po or-weather year, 30% of the females deserted their clutches, compared with 8% in a good year. Females that deserted their clutches before ha tching were significantly lighter during incubation than non-deserters , indicating that good body condition is important for successful repr oduction. 5. In one year young females lost mon mass than older female s and therefore the ability to maintain adequate body condition in the face of energetic stress appears to be age-dependent. 6. Taken togeth er, these results suggest that mass loss in breeding blue tits is, to some degree, attributable to energetic stress, although we have not ru led out the possibility that flight cost reductions may help explain t he phenomenon.