DAWSON, W. R. 



of heat production for long periods of time appears well developed 

 in many birds, particularly small northern species. These birds, 

 which include the Snow Bunting (Plectrophenax nivalis ) studied by 

 Scholander et al. (1950b), the Yellow Bunting ( Emberiza citrinella ) 

 studied by Wallgren (1954) , the several previously mentioned species 

 studied by Steen (1958), the Evening Grosbeak( Hesperiphonavesper - 

 tina), and Red and White- winged Crossbills (Loxia curvirostris ) and 

 L. leucoptera ) studiedby Dawson and Tordoff( 19 59 and unpublished) , 

 have lower critical temperatures well above the ambient tempera- 

 tures which they encounter in their habitats during winter. Although 

 it would seem advantageous for these animals to be able to supplant 

 the thermogenesis achievedby shivering with that stimulated by hor- 

 monal substances in meeting their requirements for elevated heat 

 production, they appear not to possess the latter mechanism (Hart, 

 1958). 



Once the intensification of metabolism had been achieved in the 

 avian line, relatively littlediversificationof metabolic level appears 

 to have occurred, other than that associated with the diversification 

 of body size. Although the relation of basal metabolism to body 

 weight in birds is less well known, particularly at the extremes of 

 size, and apparently more complex than that for mammals (King and 

 Earner, 1960), it appears similar in arctic, temperate, and tropical 

 species. This has led Scholander, Irving, and associates to empha- 

 size that basal metabolic rate is fundamentally no n- adaptive to cli- 

 mate (Scholander etal., 1950a; Scholander, 1955; Irving et al., 1955, 

 and Irving, 1960). 



Physical Regulation 



The various components of physical regulation, which serve to 

 alter heat lossby modification of rates of heat transfer and evapora- 

 tion, probably did not arise simultaneously in the avian evolutionary 

 line. The ability of contemporary reptiles such as the lizards Dipso- 

 saurus dorsalis and Sauromalus obesus to pant when heated (Cowles 

 and Bogert, 1944; Dawson and Bartholomew, 1958; Dill, 1938) sug- 

 gests that this mechanism for enhancing evaporative cooling could 



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