RESIDENCE IN THE ARCTIC 309 



eggs of wild birds are hatched has not been well defined. Accordingly 

 the observed duration of incubation is not a factor in the velocity 

 or chronology of embryonic growth, but only in the behavioral process 

 of incubation. 



If the conventional definition of the duration of incubation among 

 wild birds is taken to start when the last egg is laid and to extend 

 until the last egg hatches, rather consistent records are obtained for 

 each species. After examining the incubation of many species of 

 birds in equatorial Africa, the Moreaus (1940) remarked, "there is no 

 doubt that the incubation period is, within limits, specific. . . ." In 

 only a few species in tropical Africa did they find the incubation 

 period to be slightly longer than among their relatives in temperate 

 parts of Europe. 



Lack (1948) commented upon the common similarity between the 

 duration of incubation and the time which nestling birds of altricial 

 habit spend in the nest before leaving. It is certainly remarkable 

 that these two periods are often numerically similar and appear as 

 time constants of species. If incubation and nestling periods are 

 constants of species, and probably of larger taxonomic groups, they 

 are not likely to be much influenced by climate. But to seek by 

 comparisons among species whether incubation or nestling periods 

 can adapt certain species to particular climates these periods should 

 be defined in terms of developmental processes or stages in growth. 

 Otherwise we may be comparing such different stages of growth as are 

 shown in extremes by the development of precocial and altricial birds. 



Once the egg is laid, incubation provides heat, so that the tempera- 

 ture maintained during incubation is the only visible parental in- 

 fluence upon embryonic development. Kendeigh and Baldwin (1928) 

 showed that the mean temperature of the eggs of house wrens 

 {Troglodytes aedon) in nests near Cleveland was about 35° C. In the 

 same summer climate Huggins (1941) found the mean temperature 

 during incubation of 37 species of 11 orders to be 34.0° C. Among 

 the passeriform species the mean temperature during incubation was 

 33.8° C. At Cleveland, Ohio, birds of diverse phylogeny adjusted 

 their incubating behavior to the weather so as to maintain a surpris- 

 ing similarity and constancy in the temperature of their eggs. 



In summer at Anaktuvuk the temperature of the air is more variable 

 than at Cleveland. Freezing is occasional and temperatures are 

 mostly between 0° C. and 10° C. Among eggs in nests of birds of 7 

 species the median range of temperature was found to be between 

 33° C. and 35° C, with 76 percent of the records between 33° C. and 

 37° C. (L. Irving and J. Krog, 1956). We had thought that some 

 arctic nests appeared constructed for good insulation. But among 

 the eggs of a sandpiper {Ereunetes pusillus) with no nest the tem- 



