Falcon Temperature Regulation

--We measured tarsal and body temperatures of four species of large falcons in relation to rising ambient temperature and found that the tarsus has an apparent hermoregulatory function. Statistical efforts to separate the birds into ecological or plumage types yielded mixed results. An index of tarsal surface area per unit body weight was correlated with the temperature regimens of the birds, with species from the hottest climates having the greatest arsal index values. Received 28 July 1975, accepted 25 March 1977. IN 1957, Bartholomew and Cade asserted, "Despite the number of falcons which have been trained and kept in captivity, remarkably few quantitative data are available on even the most obvious aspects of their physiology" (1957, Wilson Bull. 69: 149). They examined the role of the tarsometatarsus in temperature regulation and presented evidence for a countercurrent vascular mechanism in the American Kestrel (Falco sparverius). Their conclusions led us to investigate the same mechanism in larger falcons. The genus Falco is virtually worldwide in distribution and occupies a wide range of thermal habitats. The existence of closely related forms adapted to varied environments presents an opportunity to examine adaptations of the tarsometatarsus to different thermal environments. Four species of the genus Falco were selected for study; F. peregrinus (Peregrine Falcon), F. mexicanus (Prairie Falcon), F. jugger (Lugger Falcon), and F. biarmicus (Lanner Falcon). They are of comparable size and are subject to distinctly different climates in their normal distribution. Falco peregrinus tundrius is a migratory population, breeding in subarctic regions of North America and wintering in South America. It is exposed to a moderate range of temperatures from about 25øC to -5øC. Falco p. pealei is a maritime, subarctic to north temperate year-round resident and is exposed to a narrower range of cold temperature from about 12øC to -5øC. Falco mexicanus breeds and winters over a wide range of thermal environments from arid northern Mexico to temperate Canada--a temperature range of about 42øC to -30øC. Falco jugger is restricted to the temperate environment of the northern India region and Falco biarmicus is found in arid Africa and the Middle East. Besides differences in habitat, Falco peregrinus differs from the other three species in plumage quality. Peregrines have a "hard, brittle" plumage, whereas the others have "soft, flexible" plumage. Our objective was to determine whether or not differences exist in the tarsal thermoregulatory mechanism related to the observed ecological and plumage differences.