WEST 



19 58). It has been observed that nocturnal h3qDothermia in the cold, 

 such as that recorded by Steen, in small passerines is c^ten due to 

 the birds' inability to adapt to caging and experimental conditions 

 on the first night of capture. Most birds whose body temperatures 

 dropped more than 4 C the first night of capture lost weight or 

 ultimately did not survive (Fig. 2) . 



Temporary hypothermia is common among the young of most 

 altricial species since they are essentially poikilothermic when 

 hatched and develop homeothermy during the nestling period (Bald- 

 win and Kendeigh, 1932). Body temperatures of these young, there- 

 fore, are subject to considerable variation independent of activity 

 or time of day since they are dependent for warmth on the brooding 

 of their parents. 



Heat Regulation 



Physical Mechanisms . Physical thermoregulation involves al- 

 teration in the physical aspects of the shell, increased use of the 

 respiratory surfaces as an avenue of heat loss, and changes in be- 

 havior pattern. As the temperature falls below thermoneutrality, 

 birds gradually increase their total insulation until it reaches a 

 maximum level, which is then maintained. According to classical 

 theory, this increase in insulation occurs before an increase in 

 heat production is required (Fig. 3)(Scholander et al., 1950a; Hart, 

 1957; King and Farner, 1961). Insulation in the cold involves vaso- 

 constriction of peripheral vessels, increase in the insulating ability 

 of the plumage, and behavioral adaptations such as huddling, sitting 

 on legs and feet, 'TDalling up" by putting the head under the wing, 

 burrowing, or roosting in cavities (Kendeigh, 1961a). It is evident 



that the plumage is the major insulator, since temperatures recorded 



o o 



under the skin are within 0.5 C to 1.0 C of the core temperature 



(Steen and Enger, 1957; West and Hart, unpublished), and thermo- 

 couples placed on the skin under the feathers are also within 1 C 

 to 2 C of the core temperature (Dawson and Tordoff, 1959). 



Direct measurements of the insulating ability of the plumage 

 are difficult to make (Scholander et al., 1950b), but calculations of 



294 



