THE INFLUENCE OF SEASON, TEMPERATURE, AND ABSORPTIVE STATE ON SAGE GROUSE METABOLISM

We used indirect respiration calorimetry to measure the metabolism of six adult sage grouse (Centrocercus urophasianus) during winter, sprin g, and summer. During winter the metabolic rate of fed birds was highe r (P < 0.05) than that of fasted birds. The standard metabolic rate (S MR) of females (0.692 mL O-2.g(-1).h(-1)) was higher than of males (0. 583 mL O-2.g(-1).h(-1)) in winter; in both sexes SMR was higher in win ter than in summer. Females' SMR was lower (P = 0.0001) in spring than in winter. Lower critical temperatures of both males and females were substantially lower in winter (-0.6 degrees C, -4.8 degrees C) than i n summer (14.9 degrees C, 14.8 degrees C). Although seasonally elevate d, the SMR of sage grouse in winter is low in comparison with that of other galliforms with northern distributions. Thermoregulation during a winter night at -10 degrees C would result in minimal (<5%) expendit ure of endogenous reserves by either sex. Thermoregulation and SMR in winter are more energetically costly to female sage grouse than to mal es, and may necessitate increased behavioral thermoregulation by femal es. Seasonal change in SMR differs between the sexes, and is probably influenced by the energetic demands of the breeding season.