WEST 



experiment is under way. We have observed that metabolic rates 

 remain elevated for about 1 hour after the birds have been placed 

 in a darkened metabolism chamber (West and Hart, unpublished). 



Previous thermal history affects the metabolism of an animal at 

 any given test temperature. In order to test the effect of ambient 

 temperature on mot£.oolism, it has been customary to follow one of 

 two methods: (1) Biids are acclimated to a single constant tempera- 

 ture of season and then metabolism values are obtained at a series of 

 test temperatures (Scholander et al., 1950a; Wallgren, 1954; Irving 

 et al., 1955; Steen, 1957; Dawson, 1958; Dawson and Tordoff, 1959; 

 Misch, 1960; Hart, in press; West and Hart, unpublished; and others). 

 (2) Birds are acclimated and their metabolism measured at a single 

 temperature; the temperature is changed and the birds are accli- 

 mated and run again, etc. (Kendeigh, 1949, Seibert, 1949; Davis, 

 1955; Rautenberg, 1957; West, 1960; Cox, 1961; and others). 



Many workers have assumed that a linear regression line fitted 

 to the metabolism values at a series of temperatures must extra- 

 polate to body temperature according to Newton's law of cooling 

 (Scholander et al., 1950a; Steen, 1957; and others). This interpret- 

 ation results in a distinct thermoneutral zone and a critical tempera- 

 ture which divides physical from chemical thermoregulation. Most 

 of the results on small birds obtained by these workers can be equal- 

 ly well interpreted as either a straight line drawn through all points, 

 thus eliminating the critical temperature and thermoneutral zone 

 completely (Fig. 7) or as a curve, which also eliminates the defini- 

 tion of a single critical temperature. The latter interpretation has 

 been suggested by Dawson (1958) for his data on the Cardinal (Fig. 

 8), and by Dawson and Tordoff (19 59) for the Evening Grosbeak. 



The slopes obtained by workers measuring metabolism over 

 24 hour periods are in general much flatter than those obtained in 

 short-term tests on non- acclimated birds. They show no thermo- 

 neutral zone, no critical temperature; and the temperature vs. meta- 

 bolism slope does not extrapolate to body temperature (Fig. 9). 

 Studies in progress on the Evening Grosbeak indicate that the flat 

 slopes may be explained by marked differences in diurnal and noc- 

 turnal metabolism and levels of motor activity that change with 



302 



