Dietary lipid composition and avian migratory flight performance: Development of a theoretical framework for avian fat storage.

Birds rely substantially on fat to fuel migratory flights. The importance of the composition of those fat stores to flight performance is a field of recent interest. Here I review the evidence that dietary lipid fat composition affects exercise in birds. Seasonal changes in adipose composition and diet choice experiments have yet to provide strong evidence that fatty acid composition can affect flight performance. Direct manipulations of dietary fat, however, have been demonstrated to affect exercise performance in both avian and non-avian species. I also describe the major hypotheses for the mechanisms by which dietary fat affects exercise, focusing on the role of fatty acids as oxidative substrates and as structural components of membranes. Evidence is accumulating that fatty acids that are shorter or have more double bonds increase peak performance due to their higher transport rates en route to oxidation. Endurance and efficiency of flight may or may not be affected in similar ways. Other mechanisms requiring further investigation include membrane composition, peroxisome proliferator-activated receptors, and eicosanoid-mediated inflammation. Finally, I develop a theoretical framework for studying the composition of fat stores in migrants, focusing on the tradeoffs between fatty acid transport rates, energy storage, and assimilation during stopover refueling.