Fasting in birds: General patterns and the special case of endurance flight

Abstract

Birds undergo among the most extraordinary fasting events which may last up to several months as in breeding or moulting penguins during the subantarctic winter or more than 100 h during the migratory flight of bar-tailed godwits (Limosa lapponica) from Alaska to New Zealand across the Pacific ocean. In this review we demonstrate that it is the extraordinary ability of birds to extensively store and use lipids as the main energy source which allows them to fast for long periods during breeding, moulting, and migratory flight despite their high metabolic rates. Long-term fasting birds derive the largest part of the required energy (up to about 95%) from fat and only a minimum amount from protein. In contrast to mammalian running, birds maintain such a high proportion of energy derived from fat also during endurance flight. This is achieved by physiological adaptations during preparation, as well as for the catabolism of fatty acids during flight, such as extensive lipid storage and a specific upgrading of the transport system of fatty acids to the flight muscles and their oxidative capacity. Only little is known about the hormonal regulation of the metabolism during endurance flight, most of it related to the possible actions of the glucocorticoid hormone corticosterone. There are a number of trade-offs between competing physiological processes (e.g. between maximum fat catabolism and upgrading of the metabolic system or between oxygen transport and fuel transport) which likely are linked to the migration strategy of a bird.