Body mass regulation in response to changes in feeding predictability and overnight energy expenditure

Feeding and fat storage entail both costs and benefits. Benefitsinclude minimizing the risk of starvation; costs include mass-dependentcosts of locomotion and predation risk. An understanding ofthese costs and benefits is relevant not only to explanationsof foraging patterns and fat storage, but to hoarding decisions,migration strategies, and population dynamics. Despite predictionsfrom theoretical models, empirical tests of the assumptionsand predictions of models have been tested only recently. However,published experiments on the effects of unpredictability haveoften confounded manipulations of mean, variability, and predictabilityof the food supply, all of which are predicted to affect foragingintensity and fat storage. In experiments on European starlings,Sturnus vulgaris, we manipulated the predictability of thefood supply while holding the mean and average variabilityconstant. We did this in conjunction with manipulation of overnightenergy expenditure via simulated nocturnal wind exposure. Bothgreater unpredictability of food availability and higher overnightenergy expenditure increased daily mass gain and dusk (leanand fat) mass, but in a purely additive fashion. Dawn massonly changed in response to predictability, not overnight energyexpenditure. By introducing a probe day, with identical feedingexperience for all treatments, we ascertained that the responseto predictability was based on experience integrated over morethan a single day.