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 minimizing the risk of starvation; costs include mass-dependent costs of locomotion 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. Despite predictions from theoretical models, empirical tests of the assumptions and predictions of models have been tested only recently. However, published experiments on the effects of unpredictability have often confounded manipulations of mean, variability, and predictability of the food supply, all of which are predicted to affect foraging intensity and fat storage. In experiments on European starlings, Sturnus vulgaris, we manipulated the predictability 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 unpredictability of food availability and higher overnight energy expenditure increased daily mass gain and dusk (lean and fat) mass, but in a purely additive fashion. Dawn mass only changed in response to predictability, not overnight energy expenditure. By introducing a probe day, with identical feeding experience for all treatments, we ascertained that the response to predictability was based on experience integrated over more than a single day.