Some Aspects of Temperature Relations in Small Owls

Abstract

THE owls, Strigiformes, and goatsuckers, Ca.primulgiformes, are the only widespread groups of terrestrial birds in which most species are crepuscular or nocturnal. Members of these groups exhibit morphological modifications related to their modes of life, such as enlarged, specialized eyes and the asymmetrical ears of some owls. It is of interest to speculate on possible physiological modifications in metabolic rates and temperature regulation that can be correlated with nocturnal periods of activity. Birds active at night, when temperature is at its lowest, would possibly require less special augmentation of metabolic rate, as by shivering, than diurnal species. In addition they might have low rates of metabolism that would decrease energy requirements. Nocturnal species might be expected to be less tolerant of high ambient temperatures than diurnal ones that are active during the warmer parts of the daily cycle, and also, to be less capable of withstanding extreme elevations, of body temperature. Two species of the family Caprimulgidae, the Poor-will (Phalaenoptilus nuttallii) and the Common Nighthawk (Chordeiles minor), have been carefully studied and both do, indeed show unusually low rates of metabolism (Bartholomew et al., 1962; Lasiewski and Dawson, 1964). Both species also have highly effective systems of evaporative cooling that permit them to withstand high ambient temperatures with little elevation in body temperatures (Lasiewski and Bartholomew, 1966; Lasiewski and Dawson, 1964). As similar studies dealing with the physiology of small owls are lacking, I carried out investigations on several members of this group in conjunction with my study of the Elf Owl, Micrathene whitneyi (Ligon, 1968).