310 U.S. NATIONAL MUSEUM BULLETIN 217 



perature was like that in the well insulated nest of tree sparrows. 

 It is not by nest construction but by the amount of heat contributed 

 by the incubating parent that the temperature is kept in arctic nests 

 at the same level as in temperate regions. The extra heat dissipated 

 to cool arctic air could be described in physiological terms, but the 

 uniform temperature in the nest is regulated by the behavior of 

 parent birds. 



This fact has long been known. In his famous essay Claude Ber- 

 nard (1876) referred to the even warm temperature maintained during 

 incubation as an example of regulation by behavior, and mentioned as 

 a remarkable illustration the Australian mound builder, which con- 

 structs a mound of earth and vegetation in which heat from fermenta- 

 tion supplements solar heat to maintain the correct temperature for 

 iucubation. And a recently reported measurement by H. J. Frith 

 (1956) of the temperature around the eggs of the Australian mallee 

 fowl {Leifoa ocellata) shows it to be at the common level for avian 

 incubation. The birds work over the mounds daily to modify the heat 

 conductance of the material in accord with the varying supplies of 

 solar and fermentative heat. 



Our temperature measurements among eggs of arctic birds were 

 not as well controlled as those made near laboratories in a temperate 

 climate, but the records appear no more variable than those observed 

 under better technical conditions by Kendeigh and Baldwin and by 

 Huggins. It thus seems that incubation probably does not in general 

 modify avian embryonic development through temperature, the only 

 course which we can see open. Having no measure of the progress of 

 development within the eggs I have not recorded the duration of 

 incubation. The progress of arctic reproduction suggests that its 

 essential physiology is about the same as in temperate regions, and 

 on general grounds I doubt if embryonic development can be modi- 

 fied in race or local populations as a climatic adaptation. 



Growth of Nestlings 



After the eggs hatch the parent birds might influence growth of 

 the nestlings by the temperature of brooding and by the amount of 

 food provided. Karplus (1952), observing the short duration of 

 nestling life in a brood of robins at Umiat, related it to the prolon- 

 gation of feeding in the 20-hour daily active period of the arctic 

 parents. In Swedish Lapland (lat. 68° N.) Armstrong (1954) 

 remarked upon the prolonged daily parental feeding in connection 

 with the apparent shortening of the period of nestling growth. 



There is some evidence for shortening of the duration of nestling 

 life from the tropics northward. Among 10 species of Central Amer- 



