EVOLUTION OF AVIAN TEMPERATURE REGULATION 



Incubation in birds is nicely regulated so that the eggs are maintain- 

 ed within the appropriate temperature range most of the time, de- 

 spite external conditions (compare Huggins, 1941; Irving and Krog, 

 1956; and Eklund and Charlton, 1958). Incubation is facilitated in 

 many birds by the development under hormonal control of a well- 

 vascularized and defeathered incubation patch (Bailey, 1952). The 

 uniformity of incubation temperatures for most species indicates 

 that little diversification of the thermal requirements for develop- 

 ment occurred after they were originally defined. 



Considerable diversity of parental behavior with respect to the 

 post-hatching phase of development in birds is evident, and this is 

 consistent with the wide variation in the state of maturity of the 

 young on emerging from the egg (see "Patterns in the Ontogeny of 

 Temperature Regulation"). The behavior of the parents nicely com- 

 pensates for any thermoregulatory deficiencies in the young, so that 

 development proceeds under essentially homeothermic conditions 

 (Kendeigh, 1952) independent of external temperatures. The activi- 

 ties of the parent birds include not only protecting the young from 

 cold by brooding, but also shielding them from solar radiation under 

 certain conditions, as noted in pelicans ( Pelecanus erythrorhynchos 

 and P. californicus ) and Herons ( Ardea herodias ) by Bartholomew 

 et al. (1953) and Bartholomew and Dawson (19 54a) and in Nighthawks 

 (Chordeiles minor) by Howell (1959). As young birds attain effective 

 temperature regulation, the role of parental behavior in their heat 

 economy progressively declines. 



Thermostatic Mechanisms 



It is difficult to trace the origin of the neural mechanisms con- 

 trolling temperature regulation in birds, if only because these mech- 

 anisms have thus far been characterized in only the most general 

 terms. Regulatory activity appears to be controlled principally by 

 thalamic or hypothalamic centers (Rogers, 1928; Rogers and Lackey, 

 1923), although some activity persists after these centers have been 

 eliminated (Kayser, 1929a, 1929b). In the Domestic Fowl ( Gallus 

 gallus) shivering can beelicitedby stimulation of cutaneous cold re- 

 ceptors or of central areas through reduction of skin temperature or 

 central body temperature, respectively (Randell, 1943), Panting 



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