Body temperature regulation in red-tailed hawks and great horned owls: responses to air temperature and food deprivation

-Core body temperatures of Red-tailed Hawks (Buteojamaicensis) and Great Homed Owls (Bubo virginianus) were monitored by telemetry at intervals throughout the winter and summer. The mean body temperature of the hawks over a 24-h period was 41.1PC, compared to 39.50C for the owls. Body temperature of both species fluctuated daily in association with periods of activity and rest. In the hawks, however, the difference between active and inactive body temperature increased in cold weather and in response to food deprivation because the birds maintained a lower nocturnal (inactive) body temperature. Hawks that were fasted in the winter kept their body temperatures 3.20C lower at night than during the day, which may result in a substantial savings of energy for this species at night. Body temperatures of Great Homed Owls were not influenced by cold, but were lower during inactive periods in response to food deprivation. Thus, owls may also reduce metabolic expenses during periods of food shortage by physiological adjustments. Most birds exhibit cyclical daily changes in body temperature (Tb), with highest temperatures occurring when they are active and lowest when they are inactive (see Dawson and Hudson 1970, Calder and King 1974 for reviews). A 1-30C difference in Tb between the active and inactive phases of the daily cycle may be augmented by cold exposure or food deprivation, and is especially apparent in small species (<50 g), which are incapable of subsisting on their energy reserves for more than two to three days (Calder and King 1974, Chaplin 1976, Biebach 1977, Ketterson and King 1977, Bucher and Worthington 1982). Birds of prey typically feed irregularly and may have to endure long periods of fasting during the winter when low temperatures and snow cover diminish or obscure the activity of their prey. Despite the energetic advantage gained by reducing Tb while inactive or fasting, daily fluctuations in Tb rarely exceed 1-20C in several species of owls (Coulombe 1970, Siegfried et al. 1975, Gessaman 1978), even when they are fasted to 75% of their initial body mass (Ligon 1969). Daily cycles in Tb, specifically in response to cold and/or fasting, have been reported for only a few falconiforms, and the manner in which the birds respond is still unclear. For example, Heath (1962) and Hatch (1970) disagreed as to the stability of Tb of the Turkey Vulture (Carthartes aura) when it is fasted or cold-stressed. However, the Tb of the Black Vulture (Coragyps atratus) is very labile and highly dependent on air temperature (Larochelle et al. 1982). Is such lability characteristic of falconiforms, and are the metabolic adjustments of these birds to winter therefore different from those of owls, their nocturnal counterparts? In this study, we compared the thermoregulatory responses of Red-tailed Hawks (Buteo jamaicensis) and Great Homed Owls (Bubo virginianus) to changes in air temperature and weight loss as a test of their metabolic adaptations to winter conditions. These two species are similar in size and in their utilization of habitat and prey (Craighead and Craighead 1956, Orians and Kuhlman 1956, Hagar 1957, Korshgen and Stuart 1972, McInvaille and Keith 1974, Petersen 1979, and Jaksic 1982). Their geographic ranges overlap throughout most of the United States year-round (Bent 1937, Robbins et al. 1983); even their home ranges overlap greatly in some localities (Craighead and Craighead 1956, Petersen 1979). In fact, the most notable ecological difference between them is the period of the day when they are active. Since these two species, representing different orders, are so similar ecologically, we wondered if their metabolic responses to such typical winter stresses as cold exposure and food limitation were also similar.