Metabolic Responses of Evening Grosbeaks to Constant and to Fluctuating Temperatures

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Abstract
Measurements of metabolic rates, body temperatures, and gross activity were made comparing caged evening grosbeaks (Hesperiphona vespertina) acclimatized to naturally fluctuating conditions with grosbeaks acclimated to constant temperatures in the laboratory. No significant differences were found in the relation of body weight or nighttime body temperature to constant temperature indoors or mean temperature outdoors nor between the 2 groups. Twenty-four hr. metabolic rates of both groups did not differ significantly and were linear when plotted against temperature. Nighttime rates of the 2 groups were not significantly different but were significantly lower than the daytime rates at all temperatures. No well-defined thermoneutral zone was found, and there was a gradual shift from insulative to metabolic heat regulation as temperture fell below 30[degree]C. No gross activity was detected at night during any season. Daytime activity was most frequent from 23[degree] -30[degree]C and fell off at both higher and lower temperatures. Insulation indices of both groups of birds increased with decreasing temperature during both day and night. Experiments performed on this species (and presumably other small passerines) at constant temperatures indoors adequately mimic responses of the same species under comparable but naturally fluctuating conditions outdoors.