THE ECOLOGICAL COSTS OF AVIAN FAT STORAGE

Avian fat storage is associated with both benefits and costs. Although the benefits of maintaining higher energetic reserves have long been considered, the associated costs have received far less attention. Spa tial and temporal patterns of fat storage, together with experimental data, indicate that birds are capable of actively regulating their ene rgetic reserves at levels below physiological or environmental maxima. This regulation implies that fat storage entails a cost. Evidence of potential costs are reviewed and discussed under the following heading s: mass-dependent metabolism, mass-dependent predation risk, mass-depe ndent risk of injury, mass-dependent foraging, pathological costs and reproductive costs. Although the evidence that fat storage is costly i s convincing, key empirical data are lacking. We indicate the sorts of data which need to be gathered and suggest ways in which this might b e done. We go on to discuss the interaction of these costs and their r elevance to between-individual patterns of fat storage and the interpr etation of 'condition indices'. Because many of the purported costs of fat storage are dependent upon changes in body mass, or wing loading, our review is also relevant to other phenomena which may involve mass -dependent costs, such as gonadal hypertrophy, transport of food items and primary moult.