Seasonal modulation of pectoralis muscle fiber type composition in migratory songbirds

The pectoralis major is the muscle required for migratory flight in songbirds, and has been believed to be exclusively composed of fast oxidative glycolytic (FOG) fibers in most small songbirds (< 20 g). Here, we investigated the effect of season (migratory versus non-migratory) and migratory distance (within North America versus to South America) on muscle fiber type in three songbird families: vireos (Vireonidae), warblers (Parulidae), and thrushes (Turdidae). FOG and fast glycolytic (FG) fibers were identified using myosin-ATPase staining. Short-distance migrants within the vireo and warbler families altered their pectoralis muscle to contain FG fibers during non-migratory conditions, while long-distance migrants maintained exclusively FOG fibers, regardless of season. Thrushes, a family of larger songbirds, exhibited mixed fibers regardless of season or migratory distance. This study is one of the first to identify FG fibers in small North American songbirds and highlights the potential role of migratory distance and season on muscle phenotype.