WEST 



1955, 1956; Steen and Enger, 1957; Udvardy, 19 53, 1955; Wetmore, 

 1921; and others). The variety of methods used, however, prohibits 

 legitimate comparison in most cases, e.g.: (1) use of a thermo- 

 couple or mercury thermometer inserted into the cloaca or proven- 

 triculus of a bird held in the hand, (2) use of a thermometer in the 

 cloaca or proventriculus immediately after shooting, (3) use of 

 thermocouples inserted temporarily in the cloaca, (4) use of indwel- 

 ling thermocouples implanted with the junction under the skin or in 

 the pectoral muscles. The last method will give the most satisfac- 

 tory results for comparative purposes, especially when tempera- 

 tiires are recorded continuously in the dark at night (for diurnal 

 species) while the bird is at a thermoneutral temperature (near 30 

 G) and in a post- absorptive condition (King and Farner, 1961). Deep 

 body temperatures obtained under these conditions average about 

 40.3 C for passerines and 39.5 C for non-passerines. 



The core temperatures of birds are relatively constant,, and 

 fluctuations in temperature are minimized. The shell, consisting of 

 the skin, feathers, scales, subcutaneous fat, and tissue, including 

 some skeletal muscle, acts as an insulating layer whose rate of 

 thermal conductance can be increased when deep body temper atures 

 rise and decreased when deep body temperatures fall. The distal 

 unfeathered portions of the leg and foot are most important for 

 rapid dissipation of heat (Bartholomew and Dawson, 19 58), while the 

 subcutaneous fat and feathers are important for the prevention of 

 heat loss. 



Gore temperatures of adult diurnal birds increase with gross 

 activity during the day and drop when the bird is at rest during the 

 night. At high and constant ambient temperatures, diurnal fluctua- 

 tions in body temperature are less pronounced, being about 1 C to 

 3 C (Bartholomew and Dawson, 1954; Dawson, 1954), while at low 

 ambient temperatures, body temperatures may drop 3 C to 4 C 

 at night below the normal daytime value (Fig. 1) . 



Temporary hypothermia has been recorded for adult birds in 

 the Gaprimulgiformes, Apodiformes and Goliiformes (Bartholomew, 

 Howell, and Gade, 1957) and possibly in two families of the Passeri- 

 formes, the Hirundinidae (McAtee, 1947) and the Paridae (Steen, 



292 



