RESIDENCE IN THEi ARCTIC 311 



ican Fringillidae, Skutch (1945) reported nestling life of from 12 

 to 15 days. Lack (1948) listed the duration of nestling life among 

 16 European species as being between 11 and 14i^ days. Mrs. Oake- 

 son's (1954) observations of three races of western white-crowned 

 sparrows nesting respectively at Berkeley (lat. 38° N.), Friday Har- 

 bor (lat. 49° N.), and Mountain Village (lat. 68° N.) are pertinent 

 indications that in the highest latitude the nestling period was shorter 

 in the relation 10.1, 9.6 and 9.0 days. In the duration of the nestling 

 stage at Anaktuvuk we found indications of about 10-percent shorten- 

 ing among several species of Fringillidae. This difference is not 

 related to the much greater prolongation of daily arctic feeding. 

 In fact, without some true measure of nestling development the dura- 

 tion of nestling life is only a time of parental and nestling behavior 

 and not a dimension of growth. 



We measured the temperature of nestlings in seven nests of five 

 species at Anaktuvuk (L. Irving and J. Krog, 1956). Starting with 

 the incubation temperature between 33° C. and 35° C. at hatching 

 the temperature increased but did not quite reach the temperature of 

 adult birds. In general the temperature of the nestlings was rather 

 even, for 72 percent of the records fell between 36° C. and 

 42° C. and 86 percent between 34° C. and 42° C. Wliile the mean 

 temperature of nestlings was cooler than that of adults, adult birds 

 may be 3° C. cooler at night than at rest by day and they may warm 

 3° more during activity (L. Irving, 1955). In arctic nestling 

 birds the variation in temperature little exceeds the changes in body 

 temperature which an adult usually passes through in each day. 

 The range of body temperature of the adult is regulated physiologi- 

 cally. The regulation of nestling body temperature is effected by 

 the parental behavior of brooding, although in older stages the nest- 

 lings contribute some metabolic heat and a certain amount of regula- 

 tion as they acquire insulation and control of heat production. 



It has long been known that in hot climates parent birds shield their 

 young from intense heat of the sun (Wetmore, 1921). Bartholomew, 

 Dawson, and O'Neill (1953) showed that brooding behavior was 

 accurately regulated to protect nestlings in Baja California from 

 exceeding a normal body temperature. 



The temperature requisite for growth of avian embryos or young 

 does not appear to be modifiable for adaptation to local climatic 

 conditions. Although the mean temperature during growth is lower 

 than that of the parents at rest, the variations to which the young 

 are exposed scarcely exceed the normal transition in adult body tem- 

 perature from sleep to intense activity ; and although the young lack 

 control of their body temperature, it does not seem useful to regard 



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