Seasonal Effects on Metabolism and Thermoregulation in Northern Bobwhite

Seasonal differences in metabolism and cold hardiness are common among small passerine birds. However, seasonal adjustments of metabolism and insulation are less well studied in nonpasserines and in larger birds. We measured basal metabolic rate (BMR), metabolic response to temperature, and maximal capacity for thermogenesis (peak cold-induced oxygen consumption, $\dot{{\rm V}}\text{O}_{2\ \text{sum}}$) in late spring/summer and winter in outdoor captive Northern Bobwhite (Colinus virginianus) near the northern boundary of their natural range, to determine whether seasonal adjustments in metabolism are a component of acclimatization in this species. Mass, BMR and regression equations describing diurnal and nocturnal metabolic response to temperature were not significantly different between seasons. After metabolic tests below thermoneutrality, cloacal temperature (Tb) was not dependent on ambient temperature (Ta) at either season, and nocturnal Tb did not differ significantly between seasons. However, after metabolic tests below thermoneutrality, diurnal Tb was significantly greater in summer (38.9 ± 1.1°C) than in winter (37.7 ± 1.1°C). Although body mass of winter birds was significantly greater than their body mass in late spring, maximal thermogenic capacity did not differ significantly on a seasonal basis, and winter bobwhite were only marginally more cold tolerant than late spring birds under severe cold stress. For individual birds tested in both winter and late spring, individual ranking of $\dot{{\rm V}}_{2\ \text{sum}}$ was not consistent between seasons (i.e., birds with a high $\dot{{\rm V}}_{2\ \text{sum}}$ in winter did not necessarily have a high $\dot{{\rm V}}_{2\ \text{sum}}$ in late spring). These data suggest little seasonal adjustment of metabolism or insulation in the Northern Bobwhite, a relatively large nonpasserine species native mainly to regions with relatively mild winter climates.