WEST 



ambient temperature (West and Hart, unpublished). 



It seems reasonable from the available data that most small 

 wild birds have a curvilinear relationship of metabolism to tem- 

 perature (Fig. 10). The data in Figure 4 indicate that most birds 

 increase their insulation gradually from high to low temperatures, 

 rapidly at first, then leveling off as maximum insulation is achieved. 

 Heat production, however, increases slowly at first, but then pro- 

 ceeds faster as metabolic mechanisms become the only method of 

 maintaining homeothermy at the lower temperatures. The slope of 

 the curve at the lower temperatures extrapolates to body tempera- 

 ture according to Newton's law of cooling. However, the upper por- 

 tion of the temperature- metabolism curve extrapolates beyond body 

 temperature since both insulative and metabolic mechanisms are 

 operating simultaneously. Therefore, a prolonged thermoneutral 

 zone and a definite critical temperature probably do not exist for 

 wild birds. 



Acclimation and Acclimatization 



Gelineo (1955) acclimated birds to three constant tempera- 

 tures and then obtained metabolism values at a series of test 

 temperatures for each acclimation group. In most cases the cold 

 acclimated birds had a higher metabolism slope and thermo- 

 neutral metabolism than the warm acclimated birds (Fig. 11). Sim- 

 ilar results have been obtained by Miller (1939) for House Spar- 

 rows, Dontcheff and Kayser (1934) and Steen (19 57) for the Pigeon, 

 and Wallgren (1954) for the Ortolan ( Emberiza hortulana ) and 

 Yellow Bunting ( Emberiza citrinella ) . 



Contrary to the results obtained with temperature conditioned 

 birds, most species acclimatized to summer and winter seasons do 

 not show differences in their standard metabolisms or in their 

 temperature metabolism slopes (Kendeigh, 1949, and Davis, 1955, 

 for the House Sparrow (Fig. 12); Wallgren, 19 54, for the Yellow 

 Bunting; Irving et al., 19 55, for the Black Brant ( Branta nigricans) ; 

 Rautenberg, 1957, for the House Sparrow and Brambling ( Fririgilla 

 montifrir.gilla) ; Dawson, 19 58, for the Cardinal; Hart, in press, 

 for the Pigeon, House Sparrow, Evening Grosbeak, and Starling 



306 



