ENERGETIC RESERVES OF MARSH TITS (PARUS-PALUSTRIS) - FOOD AND FAT STORAGE IN RESPONSE TO VARIABLE FOOD-SUPPLY

During the winter, small birds face the threat of starvation during the day and overnight. The threat of starvation can be reduced by maintaining higher energetic reserves, either as internal fat stores or as external hoards. I present here empirical data concerning the management of energetic reserves in response to unpredictability in foraging success. Predictions of a model were tested using marsh tits (Parus palustris) experiencing conditions of low and high temporal variation in food supply. Under the high-variation treatment, birds increased external hoards but not internal fat reserves. In addition, birds decreased their mean hoard-recovery interval compared to conditions in which food was predictable. Detailed examination of body weight throughout the day indicates that marsh tits delayed the accumulation of overnight fat reserves until relatively late in the day. Overall, these results support the theory that fat reserves are costly to acquire and maintain and suggest that marsh tits actively manage both internal and external energetic reserves within and among days.