Seasonal Metabolic Acclimatization Varies in Direction and Magnitude among Populations of an Afrotropical Passerine Bird

Avian metabolic responses demonstrate considerable diversity under fluctuating environmental conditions, a well-studied example being the seasonal upregulation of basal metabolic rate (BMR) and summit metabolism (M-sum) in temperate species experiencing harsh winters. Fewer studies have examined seasonal metabolic acclimatization in subtropical or tropical species. We investigated seasonal metabolic variation in an Afrotropical ploceid passerine, the white-browed sparrow-weaver (Plocepasser mahali; approximate to 47 g), at three sites along a climatic gradient of approximately 7 degrees C in winter minimum air temperature (T-a). We measured M-sum (n 10 per site per season) in a helox atmosphere, BMR of the same birds at thermoneutrality (T-a approximate to 30 degrees C), and resting metabolic rates at 5 degrees C T-a 20 degrees C. Patterns of seasonal adjustments in BMR varied among populations in a manner not solely related to variation in seasonal T-a extremes, ranging from BMR approximate to 52% higher in winter than in summer to no seasonal difference. Greater cold tolerance was found in a population at a colder desert site, manifested as higher M-sum (approximate to 25% higher) and lower helox temperature at cold limit values compared with a milder, mesic site. Our results lend support to the idea that greater variance in the pattern of seasonal metabolic responses occurs in subtropical and tropical species compared with their temperate-zone counterparts and that factors other than T-a extremes (e.g., food availability) may be important in determining the magnitude and direction of seasonal metabolic adjustments in subtropical birds.