LIKNES, E.T.; SWANSON, D.L.; Northern State University; University of South Dakota: Phenotypic flexibility of body composition associated with seasonal acclimatization of resident passerines
Improved winter cold-tolerance is widespread among small birds from temperate climates and is usually associated with improved shivering endurance and elevated peak metabolic rate (Msum), although acclimatization mechanisms are incompletely understood. Seasonal phenotypic flexibility of skeletal muscle and nutritional organs associated with seasonal variations in Msum and basal metabolic rate (BMR) is a pattern predicted by the Variable Maximum Model (VMM) model of seasonal acclimatization. To test this model, we investigated seasonal changes in body composition of three resident passerine species with characteristically large seasonal variations in Msum: White-breasted Nuthatch (Sitta carolinensis), Black-capped Chickadee (Poecile atricapillus), and House Sparrow (Passer domesticus). All species displayed significant changes in body composition from summer to winter. Chickadees and nuthatches increased pectoralis, heart and some nutritional organ masses in winter. Sparrows showed only increased heart and nutritional organ masses in winter. In all species, some organs masses were also influenced by sex. Seasonal changes in heart and pectoralis masses appear to contribute importantly to thermogenic capacity in small birds while changes in nutritional organ masses may reflect improved energy assimilation capacity and/or changes in diet. We believe these data are consistent with the VMM model of seasonal acclimatization.
