Patterns of Lipid Storage and Utilization in Birds

Abstract

Nowhere among the vertebrates does the capability for storing and using triglyceride as an energy reserve exceed the level found in the class Aves. Adult avian depot fats are composed largely of 16- and 18-carbon fatty acids and are mostly unsaturated. Variation in fatty acid composition among species may be attributed to dietary differences and physiological state of the bird. Storage occurs mainly by addition of lipid to adipocyte vacuoles without an increase in cell number. Daily cycles of fat deposition and use are of greater amplitude at higher latitudes, but in general the lipid stored during the day will only provide energy for the overnight fast plus a few daylight hours. Storage levels may be minimized due to the disadvantages of increased wing-loading. A variety of behavioral, physiological and morphological adaptations may be used to reduce the need for overnight energy reserves. Migratory fattening is largely a function of migration speed, magnitude of barriers to be crossed and aerodynamic considerations. Lipid reserves are greatest in eggs of precocial birds and are retained longer in precocial young. Adaptive strategies of fat deposition in young birds are related largely to the ability of adults to feed young before and after fledging.