DAV/SON, W. R. 



Undoubtedly the first homeotherms in the avian line received a 

 considerable legacy of thermally significant behavioral patterns 

 from their reptilian antecedents. Indeed, the evolution of physiologi- 

 cal mechanisms for regulation of body temperature may well have 

 been originally concerned with augmenting thermoregulatory be- 

 havior. As physiological capacities for temperature regulation im- 

 proved, behavior came to occupy the ancillary role in the manage- 

 ment of the heat economy evident in birds today. Many species, as 

 a result of their migratory habits, are able to exploit various en- 

 vironments on a seasonal basis and to evade unfavorable conditions 

 to a great extent. Birds resident in hot climates modify the impact 

 of their environments to some extent by utilizing shade, minimizing 

 activity, and, in some cases, bathing during the heat of the day (Daw- 

 son, 1954). In a few instances birds resident in cold climates also 

 employ behavioral mechanisms incopingwithwinter conditions. For 

 example, at night ptarmigan (Lagogus) utilize the shelter afforded by 

 burrows in the snow (Irving, 1960) and Creepers ( Certhia brachyda - 

 tyla) huddle together in bunches of 10 to 20 (Lohrl, 19 55). In general 

 it appears that birds, particularly the smaller ones, are less suc- 

 cessful in evading the extremeconditionsof their environments than 

 their mammalian counterparts. The fact that most desert birds are 

 diurnal and fail to take advantage of the shelter afforded by under- 

 ground burrows forces them to contend with heat as well as aridity. 

 This has an important effect on their water economies because it 

 requires rapid rates of evaporative water loss (Bartholomew and 

 Dawson, 1953; Bartholomew and Cade, 1956; Dawson, 1958). Such 

 is not the case in most small desert mammals, which are fossorial 

 and nocturnal (see Schmidt- Nielsen and Schmidt- Nielsen, 19 52). 

 The failure of birds in cold climates to utilize underground bur- 

 rows and nests also deprives them of effective means of protec- 

 tion utilized by many mammals. 



With the establishment of homeothermy in the avian line, the 

 general thermal requirements for development became restricted 

 to a fairly narrow range of temperatures a few degrees below the 

 level of body temperature in adults. This restriction must have been 

 accompanied by the evolution of elaborate patterns of parental be- 

 havior, which are evident in contemporary birds (Kendeigh, 1952). 



50 



