Seasonal Changes in Composition of Lipid Stores in Migratory Birds: Causes and Consequences

It is well established that birds use fat stores to primarily fuel migration; however, few studies have focused on the causes and consequences of observed seasonal changes in fatty acid composition of fat stores in birds. We propose and test two hypotheses that address the causes of these seasonal changes in composition of fat stores: (1) diet composition determines fatty acid composition of fat stores, and (2) birds selectively metabolize and store certain fatty acids during migration in lieu of changing their diet. When we offered Red-eyed Vireos (Vireo olivaceous) choices between diets that differed only in fatty acid composition, vireos preferred diets with more triolein over diets with more tristearin and tripalmitin, and these preferences were similar between seasons. We also collected fat samples six times throughout the year from captive Red-eyed Vireos fed one of two diets differing in fatty acid composition, and found that fatty acid composition of stored fat differed by diet and changed over time, although these changes were not season-specific or consistent with the selective-metabolism hypothesis. Thus, fatty acid composition of stored fat was primarily a product of diet composition; selective metabolism possibly played a minor, but important, role. Given recent evidence that fatty acid composition of birds affects their energy expenditure during intense exercise, the implication is that birds at stopover sites can influence the fatty acid composition of their body fat by selective feeding, and this can significantly affect the energetic cost of migration.