The effect of social dominance on fattening and food-caching behaviour in Carolina chickadees, Poecile carolinensis.

Subordinates often have to wait for dominants to obtain food. As a result, their foraging success should be less predictable and they should therefore maintain a higher level of energy reserves compared with dominants. A corollary of this prediction is that subordinates should gain mass earlier in the day and maintain higher mass than dominants. We tested these predictions with captive Carolina chickadees. In two different experiments (one where birds were given ad libitum access to food and the other with food access limited to 60 min/day), we formed social flocks of two previously unfamiliar birds and compared their energy management (body fat and food caches) while they were in the flock with energy management when housed alone. Results from both experiments failed to support the predictions. Of all the parameters of body mass and food caching we measured only the following results were significant: (1) On the ad libitum food schedule, both subordinates and dominants accumulated more mass over the day when in a flock compared with when they were solitary, and there were no differences in mass gain between dominants and subordinates. (2) When analysed separately, dominants showed a higher evening mass in the flock compared with the solitary condition, a trend that runs opposite to the prediction. Our results suggest that when in favourable foraging conditions, social interactions might cause dominant and subordinate birds to accumulate more energy reserves as a result of competition. On the other hand, if food supply is limited, both dominants and subordinates may be forced to maintain similar fat reserves as an insurance against increased risk of starvation. Copyright 2000 The Association for the Study of Animal Behaviour.