An Analysis of the Body Temperatures of Birds

Recently there has been much work done on the energetics of homoiotherms. One parameter of energetics, body temperature, is relatively easy to measure and, thus, has been often studied. An analysis of the body temperatures of mammals was proposed by McNab (1966), but none has been made of bird temperatures. The purpose of this report is to analyze the factors responsible for the level of body temperatures of birds. Two conclusions on bird temperatures can be drawn from the available data (table 1) : (1) they are almost always above those of mammals, and (2) they are low in ratities and penguins, high in passerines, and intermediate in other birds. These differences have no obvious explanation. Usually the difference in body temperature between mammals and birds is ignored. If an explanation is offered, it generally consists of the correlation that flying birds have higher body temperatures than mammals or flightless birds. Why this should be, however, is not clear, since the low body temperatures of temperate-zone bats indicate that a high resting body temperature is not a rigid requirement for flight. Rodbard (1950) suggested that small birds have higher body temperatures than small mammals because feathers are better insulators than hair; this suggestion will be examined later. The correlation of body temperature with taxonomic group is either ignored or the suggestion is made that it represents the evolutionary attainment of good thermoregulation (Sutherland, 1899 ; Wetmore, 1921). This explanation disregards the correlation of body temperature with weight. Large birds tend to have low body temperatures and small birds, high body temperatures (Rodbard, 1950) ; this correlation, in itself, requires an explanation, since there is apparently no dependence of body temperature on weight in mammals (Morrison and Ryser, 1952), in spite of a proposal to that effect by Rodbard (1950).