We investigated the relationships between social dominance, competition for food, and strategies of body mass and fat regulation in the European starling (Sturnus vulgaris). In birds housed in groups of three, subdominant birds stored more fat than dominants. A removal experiment established a causal link between social dominance and fat reserves; in groups that had the dominant individual removed, the remaining birds reduced body mass and fat, relative to control groups that had the subordinate removed. In a second experiment, we investigated the influences of degree of competition for food and dominance on body mass and fat reserves. Birds under high competition increased fat reserves and tended to have higher body mass than birds under low competition. The increase in fat reserves was higher in the subdominants than in the dominants. These results are consistent with hypotheses concerning dominance-dependent access to food; subdominant birds, or birds under increased competition, may store more fat as an insurance against periods when food cannot be obtained. However, relations between dominance, body mass, and fat reserves may also arise through other proximate factors relating to dominance-dependent costs and benefits of fat storage, such as predation risk and energetic expenditure.

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