Phenotypic flexibility in passerine birds: Seasonal variation of aerobic enzyme activities in skeletal muscle

Improved winter cold tolerance is widespread among small passerines resident in cold climates and is generally associated with elevated summit metabolic rate (M_sum=maximum thermoregulatory metabolic rate) and improved shivering endurance with increased reliance on lipids as fuel. Elevated M_sum and improved cold tolerance may result from greater metabolic intensity, due to mass-specific increase in oxidative enzyme capacity, or increase in the masses of thermogenic tissues. To examine the mechanisms underlying winter increases in M_sum, we investigated seasonal changes in mass-specific and total activities of the key aerobic enzymes citrate synthase (CS) and β-hydroxyacyl CoA-dehydrogenase (HOAD) in pectoralis, supracoracoideus and mixed leg muscles of three resident passerine species, black-capped chickadee (Poecile atricapillus), house sparrow (Passer domesticus), and white-breasted nuthatch (Sitta carolinensis). Activities of CS were generally higher in winter than in summer muscles for chickadees and house sparrows, but not nuthatches. Mass-specific HOAD activity was significantly elevated in winter relative to summer in all muscles for chickadees, but did not vary significantly with season for sparrows or nuthatches, except for sparrow leg muscle. These results suggest that modulation of substrate flux and cellular aerobic capacity in muscle contribute to seasonal metabolic flexibility in some species and tissues, but such changes play varying roles among small passerines resident in cold climates.