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. Spatial and temporal patterns of fat storage, together with experimental data, indicate that birds are capable of actively regulating their energetic 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 headings: mass-dependent metabolism, mass-dependent predation risk, mass-dependent risk of injury, mass-dependent foraging, pathological costs and reproductive costs. Although the evidence that fat storage is costly is convincing, key empirical data are lacking. We indicate the sorts of data which need to be gathered and suggest ways in which this might be done. We go on to discuss the interaction of these costs and their relevance to between-individual patterns of fat storage and the interpretation 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.