Environmental and Social Determinants of Winter Fat Storage in the Great Tit Parus major

Abstract

1. It has generally been assumed that reduced winter fat reserves in small birds are symptomatic of environmental constraints on their ability to fatten further. Contrary to this, recent theoretical work on foraging and fat storage suggests that such observations result as a strategic response to environmental stress, determined by a balance of fattening costs and benefits that differ between individuals. Fat reserves should therefore be adjusted to the minimum required as insurance against some period of stress, but birds should respond to the resource variance (predictability) as well as the mean abundance. Hence, birds facing greater starvation risk should carry more fat than those facing less risk. 2. These hypotheses predict that, in a winter-flocking species with a clear dominance hierarchy, and in which social status may determine a bird's access to food, individuals of different social status should differ in the fat reserves that they carry as follows: (i) fat should increase with declining temperature; (ii) the rate of diurnal fattening should be reduced when food is abundant; (iii) fat reserves should be increased when food is scarce; and (iv) dominant individuals should carry less fat than subdominants. This paper tests these predictions by observations of fat reserves of live great tits within a marked, free-living population in Wytham Woods, Oxford, UK. It then examines the relationship between the winter fat and over-winter survival of known individuals. 3. Fat increased with declining temperature on the day of capture rather than at any previous time, suggesting that it acted as a proximate factor in fat regulation. The rate of fattening was lower when food was more predictable. Reserves increased when food was scarce but this depended on the bird's status, dominants carrying less fat than subdominants. Evidence that the ability to fatten might be subject to external constraint was found only in the least dominant individuals. 4. When food was abundant, fat reserves failed to predict over-winter survival, suggesting that the birds were able to maintain their individual optimum reserves. When food was scarce, low ranking birds were more likely to survive if fat, while the survival of dominants was independent of fatness, suggesting that they were still able to achieve their individual optimum reserves. 5. These results agreed with the predictions, so supporting the arguments (i) for a cost of fattening, (ii) that fattening follows a strategic response to resource predictability; and (iii) that social status is a component in that resource predictability.