||Skeletal muscle is the primary site of thermogenesis in birds. The relative contributions of shivering and non-shivering thermogenesis (ST and NST, resp.) are unknown. Morphological changes can help distinguish the relative importance of ST and NST during development and acclimation. This study assessed growth and several ultrastructural characteristics of pectoralis (PM) and iliotibialis (IL) muscles in Japanese Quail acclimated to cold (5°C) (cold-acclimation; CA) and warm temperatures (25°C) (warm-acclimation; WA) for three weeks starting at age 14d. Birds in a third group were dissected prior to acclimation at age 14d. CA birds maintained lower body temperatures and allocated more energy to thermogenesis. Fiber and myofibril size generally changed more with age than acclimation, as did mitochondrial area (as % fiber area) in the PM, contrary to predictions. The subsarcolemmal mitochondria (SSM) increased disproportionately relative to the intermyofibrillar mitochondria (IFM) in both CA and WA birds, suggesting this subpopulation remains an important site of catabolism. Intermuscular differences were as expected with changes in aerobic capacity. Mitochondria of the PM had more cristae, especially in SSM of CA quail, suggesting higher catabolic capacities to support both elevated ST and NST. These data suggest that CA quail adapt via regulated hypothermia and quantitative changes within mitochondria.