WEST 



-30 -20 -10 10 20 30 40 

 TEMPERATURE *C. 



Figure 4. Insulation indices calculated by the formula: 



Body T C - Air T G 

 kcalA)ird/hour 



Average indices of birds held at constant temperature (•,A); and of birds under 

 natural fluctuating conditions (0,A). TS=Tree Sparrow (West, 1960), GB=Evening 

 Grosbeak (West and Hart, unpublished). Thedashed lines are values calculated from 

 the literature: J=Slate-colored Junco (Seibert, 1949); YB=Yellow Bunting (Wallgren, 

 1954); VSE=Variable Seedeater (Cox, 1961; C=Cardinal (Dawson, 1958); HS=House 

 Sparrow (Kendeigh, 1949); BJ=Blue Jay (Misch, 1960); GB=EveningGrosbeak (Dawson 

 and Tordoff, 1959). 



298 



