Responses of Body Mass, Organ Masses, and Metabolic Rates in Winter-Phenotype House Sparrows to Fluctuating Cold Temperatures

AbstractSmall birds in temperate regions are faced with a large range of environmental conditions throughout the year, including fluctuating temperatures. During cold winters, birds often exhibit an increase in metabolic rates, body mass, and pectoralis muscle mass because of the heightened energetic needs of thermoregulation. However, climate change is altering weather patterns, and in addition to widespread winter warming, temperature variability and the frequency of extreme temperatures are also expected to increase, including more winter cold snaps. In the present study, our goal was to determine whether an increase in temperature variability in a cold environment will impact the metabolic rates, organ masses, and body mass of winter-phenotype house sparrows (<i>Passer domesticus</i>). After exposing birds to stable warm, stable cold, or fluctuating cold temperatures, we found no significant differences in masses or metabolic rates between the stable and fluctuating cold groups. Compared to the warm treatment, both cold treatments had higher basal, but not summit (i.e., maximum, cold induced), metabolic rates. These results suggest that increasing temperature variability may not influence the maintenance costs or the thermoregulatory capacity of winter-phenotype house sparrows.