Physiological Thermoregulation in Heat-Stressed Birds: Consequences of Body Size

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Abstract
Metabolic heat production ( ), evaporative heat loss ( ), and body temperature ( ) were measured simultaneously in six species ranging in size from 12 to 125 g. These data, together with those of previous studies, indicate that is not maintained constant at ambient temperatures ( ) below the upper critical temperature ( ), but increases linearly with above about 30 C. Consequently, increases within the thermoneutral zone while remains unchanged. Plotting within the thermoneutral zone as a function of indicates that metabolism is independent of (i.e., ) in birds smaller than about 100 g. Calculations of rates of heat exchange show that hyperthermia contributes little to the cost of avian temperature regulation in hot environments, while it significantly enhances water conservation. In all species increased linearly with increasing above . The slope of the line relating to above is an integrated measure of the cost of thermoregulation in hot environments, here designated as the coefficient of heat strain ( ). In 26 species of birds, (mW gl⁻¹C⁻¹) varies with size according to the relation log , where m is body mass in grams. At the size of the gradient was inversely related to body size, decreasing from 5.0 C in the 12-g blackthroated sparrow (Amphispiza bilineata) to 1.2 C in the 125-g Gambel's quail (Lophortyx gambelii). Consequently, small birds depend less on evaporative cooling at than large birds, a circumstance favoring water economy in small birds. All species were able to increase during heat exposure without increasing . The extent to which increased without a concomitant increase in was, however, inversely related to body size. This may reflect a greater capacity to alter the plumage vapor gradient in larger birds.