312 U. S. NATIONAL MUSEUM BULLETIN 217 



them as poikilothermous when they live with and probably require 

 about the same uniformity of temperature as their parents. 



Since this sort of homoiothermism seems to be innate in the embryo, 

 which originates from cells in a warm parent, there appears little 

 opportunity for the environment to modify avian development through 

 the influence of temperature. At present this opinion is speculative 

 because it is based only upon observations of time and temperature 

 without support from measurements of differentiation or growth. 



Among 28 species for which I can prepare schedules of migration 

 and nesting at Anaktuvuk the first eggs, as shown in table 14, were 

 found between May 23 and June 10. The latest fresh eggs found in 

 this latitude were in a robin's nest on July 21 (L. Irving and Paneak, 

 1954) . Late in June I have found a few pairs of redpolls beginning 

 to lay. The Nunamiut tell me that among the bare willows in winter 

 they occasionally find nests containing frozen emaciated nestling red- 

 polls. Starvation had preceded freezing. The Eskimo view is that 

 as August progresses redpoll parents become so anxious to join a flock 

 that they lose interest in family affairs. This good subjective descrip- 

 tion of the situation shows that it is not the cold weather which marks 

 the end of parental care but the onset of molt or other internal changes 

 which are represented in the social transition from family attachments 

 to the formation of flocks. 



It seems clear that young and adult birds must prepare in summer 

 for the change of social regime which goes with the change of 

 season. In order to leave time enough for the late summer prepara- 

 tions for change of habit, residence, and season, arctic birds must pass 

 rapidly through the stages of the reproductive cycle occurring between 

 migration and nesting. 



Departure 



The departure of most species of birds from Anaktuvuk in late sum- 

 mer is not easy to record accurately. The termination of concern for 

 breeding first becomes obvious among some species of ducks, like 

 scaup and pintails, when the male birds begin to assemble in flocks in 

 mid-July. By the end of July the young of most of the passerif orm 

 birds are flying. Family associations then relax and the birds of some 

 species begin to assemble in more or less well organized flocks. Other 

 species, like tree sparrows, withdraw from view and are infrequently 

 seen. Some conspicuous associations before migration southward can 

 be seen in the following examples : 



