Physiological flexibility and its impact on energy metabolism and foragingbehaviour in birds

Many aspects of foraging, for instance locomotion, social interactions, dig estion, but also maintenance, require the expenditure of energy, and theref ore the metabolism of food and energy reserves. In most current models of f oraging behaviour, the individual's physiology is taken to be static. Howev er, recent work shows that it may vary in response to changes in diet, food intake rate, predictability of food resources and temperature. In addition , during the annual cycle an individual may change its body composition in anticipation of reproduction, migration and wintering. Virtually all organs and reserve depots can be involved in these alterations. Focusing on (mono gastric) birds, this potential for flexibility and its boundaries are revie wed with regard to their impact on energy expenditure. It is argued that th ese energy budget modifications may have distinguished consequences for for aging and habitat use.