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

Feeding and fat storage entail both costs and benefits. Benefits include mi nimizing the risk of starvation; costs include mass-dependent costs of loco motion and predation risk. An understanding of these costs and benefits is relevant not only to explanations of foraging patterns and fat storage, but to hoarding decisions, migration strategies, and population dynamics. Desp ite predictions from theoretical models, empirical tests of the assumptions and predictions of models have been tested only recently. However, publish ed experiments on the effects of unpredictability have often confounded man ipulations of mean, variability, and predictability of the food supply, all of which are predicted to affect foraging intensity and fat storage. In ex periments on European starlings, Sturnus vulgaris, we manipulated the predi ctability of the food supply while holding the mean and average variability constant. We did this in conjunction with manipulation of overnight energy expenditure via simulated nocturnal wind exposure. Both greater unpredicta bility of food availability and higher overnight energy expenditure increas ed daily mass gain and dusk (lean and fat) mass, but in a purely additive f ashion. Dawn mass only changed in response to predictability, not overnight energy expenditure. By introducing a probe day, with identical feeding exp erience for all treatments, we ascertained that the response to predictabil ity was based on experience integrated over more than a single day.