EVOLUTION OF AVIAN TEMPERATURE REGULATION 



young of precocial species, e. g., gallinaceous birds, which are 

 hatched at a relatively mature state and soon afterward develop such 

 regulation. Indeed, several observations indicate that some ability 

 for temperature regulation is present in precocial birds even before 

 hatching. Between the extremes represented by typically altricial 

 and precocial birds are many species, e. g., cap rimulgids, which are 

 intermediate in their developmental state at hatching. 



The precocial condition is assumed to be primitive in birds 

 (Kendeigh, 1952). Evolution of the altricial condition has been keyed 

 to the elaboration of patterns of parental behavior. Its appearance 

 has been considered important from the standpoint of bioenergetics. 

 The immature state ofnewly hatched altricial young and the relative- 

 ly short period between fertilization and hatching allows a smaller 

 egg of lower energy content than is generally found in precocial birds 

 of similar adult size (Huxley, 1927). Therefore less demand is made 

 on the energy resources of altricial females per egg produced. The 

 fact that the young do not develop beyond a very immature state in 

 the egg is compensated for by parental activity in their care and 

 feeding. In many species this burden is shared by both parents. Par- 

 ental behavior is effective in maintaining the young at near-homeo- 

 thermic levels of body temperature before their powers of tempera- 

 ture regulation become established. Thus they are able to develop 

 under favorable conditions without having to expend energy beyond 

 basic maintenance and developmental needs. In passerines, at least, 

 the energetic obligations of homeothermy are only assumed when the 

 young are nearing mature size (Kendeigh, 1939; and Dawson and 

 Evans, 19 57, 19 60). On the other hand, precocial young, although they 

 too may be brooded or may huddle with their siblings in cool envi- 

 ronments (Lehmann, 1941; and Kleiber and Winchester, 1933) , must 

 rely on their own energy to a considerable degree for growth and 

 development and for maintenance of body temperature once they are 

 hatched (Bartholomew and Dawson, 19 54a). 



It has been suggested (Kendeigh, 1952; and Witschi, 1956) that 

 the evolution of small birds was in part made possible by the devel- 

 ment of the altricial mode ofdevelopment. Certainly, the lower limit 



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