Migration-associated changes in hormones and muscle physiology of migratory redheaded buntings (Emberiza bruniceps)

Long distance migration in birds requires dynamic remodelling of flight muscle induced by behavioural, endocrine and metabolic adjustments. Here, we studied redheaded buntings in the nonmigratory and migratory state to assess changes in plasma hormones, muscle fibre diameter, lipid droplet accumulation and gene expression in the pectoralis and supracoracoideus muscles. We investigated genes that are involved in (1) muscle growth and atrophy, and (2) fatty acid transport and metabolism. Long-day exposure induced nighttime migratory restlessness, increased body mass, fat and muscle scores, and enlarged testes, compared with short days. Plasma insulin and testosterone levels were elevated in the migratory group, whereas T3 levels remained unchanged. Only the pectoralis muscle showed significant hypertrophy and high lipid accumulation under long days, whereas the supracoracoideus remained unchanged. We found elevated mRNA levels of fabp3 (fatty acid binding protein) and ar (androgen receptors) in both muscles, but scd1 (stearoyl-coA desaturase 1) only in the pectoralis. In contrast, igf1 (insulin-like growth factor 1) mRNA levels decreased in pectoral muscle in migratory state. These findings suggest that although fabp3 may facilitate fatty acid transport to both muscles, scd1 contributes to lipogenesis in pectoral muscle only. This suggests that pectoral muscles play a more active role than supracoracoideus muscle in uptake, modification and storage of lipids during migration. These findings demonstrate muscle-specific physiological remodelling underlying the migratory phenotype in buntings. Network analysis further revealed greater integration of phenotype, histology, hormone and molecular traits during migration, suggesting enhanced coordination of physiological systems during migration.