PPAR expression, muscle size, and metabolic rates across the Gray catbird's annual cycle are greatest in preparation for fall migration

Phenotypic flexibility across the annual cycle allows birds to adjust to fluctuating ecological demands. Varying energetic demands associated with time of year have been demonstrated to drive metabolic and muscle plasticity in birds, but it remains unclear what molecular mechanisms control this flexibility. We sampled Gray catbirds at five stages across their annual cycle: tropical wintering (January), northward spring (late) migration (early May), breeding (mid June), the fall pre-migratory period (early August), and southward fall (early) migration (end September). Across the catbird's annual cycle, cold-induced metabolic rate (V̇o2summit) was highest during migration and lowest during tropical wintering. Flight muscles exhibited significant hypertrophy and/or hyperplasia during fall migratory periods compared to breeding and the fall pre-migratory period. Changes in heart mass were driven by the tropical wintering stage where heart mass was lowest. Mitochondrial content of the heart and pectoralis remained constant across the annual cycle as quantified by aerobic enzyme activities (CS, CCO), as did lipid catabolic capacity (HOAD). In the pectoralis, transcription factors PPARα, PPARδ, and ERRβ, coactivators PGC-1α and β, and genes associated with fat uptake (FABPpm, Plin3) were unexpectedly upregulated in the tropical wintering stage and genes involved in fatty acid oxidation (ATGL, LPL, MCAD) were downregulated suggesting a preference for fat storage over utilization. Transcription factors and coactivators were synchronously up-regulated during pre-migration and fall migration periods in the pectoralis but not the heart, suggesting these pathways are important in preparation for and during early migration to initiate changes to phenotypes that facilitate long distance migration.