Metabolic characteristics and body composition in house finches: effects of seasonal acclimatization

Abstract

House finches (Carpodacus mexicanus) from the introduced population in the eastern United States were examined to assess metabolic characteristics and aspects of body composition associated with seasonal acclimatization. Wild birds were captured during win- ter (January and February) and late spring (May and June) in southeastern Michigan. Standard metabolic rates did not differ seasonally, but cold-induced metabolic rate was 28% greater in winter than late spring. The capacity to maintain elevated metabolic rates during cold exposure (thermogenic endurance) increased significantly from an average of 26.1 to 101.3 rain in late spring and winter, respectively. House finches captured in the late afternoon during winter had twice as much stored fat as those during late spring. Both the wet mass and lean dry mass of the pectoralis muscle, a primary shivering effector, were significantly greater during winter. The seasonal changes in peak metabolism and thermogenic endur- ance demonstrate the existence and magnitude of meta- bolic seasonal acclimatization in eastern house finches. Increased quantities of stored fat during winter appear to play a role in acclimatization, yet other physiological adjustments such as lipid mobilization and catabolism are also likely to be involved.