EVOLUTION OF TEMPERATURE REGULATION 

 IN BIRDS 



William R. Dawson* 



The possession of homeothermy by birds and mammals has ex- 

 ercised a major influence on their evolution, both through the bio- 

 logical opportunities it has afforded and through the physiological de- 

 mands it has imposed. The evolutionof the mechanisms responsible 

 for this condition merits consideration not only because of its im- 

 portance to these groups of vertebrates, but also because it com- 

 prises a major step in a general trend within the Animal Kingdom 

 toward increasing control of internal state. The present discussion 

 will deal primarily with the evolution of the mechanisms of tempera- 

 ture regulation in birds, although reference will be made to mam- 

 mals where comparisons are appropriate. The development of tem- 

 perature regulation in this latter group is treated in detail elsewhere 

 (Johansen, 1962). 



THE HISTORICAL BACKGROUND 



Current concepts of the origin and early deployment of birds are 

 largely a matter ofdeduction, owing to the very incomplete fossil re- 

 cord. The structure of the earliest known bird. Archeopteryx litho - 

 graphic a . from the upper Jurassic of Bavaria, places the origin of 

 the class among the thecodont reptiles (Swinton, 1960). Birds appear 

 to have arisen from a single line which appeared with the radiation 

 of this reptilian order in the Triassic. The stage in the development 

 of this line at which homeothermy was achieved is unknown, and for 

 this reason subsequent references to the establishment of this condi- 

 tion in the "avian evolutionary line" are intentionally vague. Swinton 

 (1960) suggests that the immediate antecedents of birds were arbore- 

 al and at least partially homeothermic, and that true flight was not 



♦Preparation of this paper was supported in part by a grant from the National 

 Science Foundation (G-9238). 



45 



