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U. S. NATIONAL MUSEUM BULLETIN 217 



evidence. I do not suspect that incubation can be environmentally 

 modified and I would be surprised if nestling growth could be signifi- 

 cantly modified by environment (L. Irving and Krog, 1956). The 

 conspicuous influence upon the breeding cycle appears in the synchro- 

 nization of events so that their succession is swift in the entire popu- 

 lation. The adaptation of breeding to the arctic environment thus 

 appears as a social phenomenon rather than as a modification of the 

 individual's physiological processes, many of which are so deeply 

 rooted in the origins of the species that their adaptive modifiability 

 to local conditions is hardly to be suspected. (For a further discus- 

 sion of this subject see also chapter 7.) 



In six of the nests where complete clutches were apparent the aver- 

 age of the numbers of eggs was 5.5.. I think it proper to regard the 

 set of three nestlings as diminished by casualty. At Anaktuvuk sev- 

 eral complete clutches contained either five or six eggs. Mrs. Oakeson 

 kindly permits reference to her comparison of clutch size among white- 

 crowned sparrows in various localities : 



Bace 



nuttalli 



pugetensis 



gambelii 



gambelii 



gambelii 



Our observations do not have the significance of her more careful 

 studies, but they show that the number of eggs laid near the arctic 

 limit of the range of white-crowned sparrows is largest. We would 

 not be inclined to relate this northward increase to latitude, for the 

 increase of eggs is not proportional to the differences in latitude. We 

 have earlier remarked that in many species, like plovers and sand- 

 pipers, the size of clutch is invariable, and the influence of environ- 

 ment upon size of clutch is not general. The large number of eggs 

 laid is, however, certain evidence of the adequacy of nutrition near 

 Old Crow in a dense population of white-crowned sparrows. The 

 rarity of casualties observed in nests is another indication of the suc- 

 cessful adaptation of breeding habits to the locality. 



Passerella iliaca zaboria Oberholser 



Location 



Clutches 



Average 

 size 



Central California 



147 



3.25 



Seattle, Friday Harbor, Victoria 



44 



4.09 



Mountain Village 



6 



4.7 



College 



13 



4.9 



Old Crow 



6 



5.5 



25 males 



May 14-Jmie 20 



Weight, average 

 (25), 35.9 g., 

 coefE. of var. 5.8% 



7 females 



May 16-June 20 



Weight, average 

 (3), 34.3 g. 



2 young males 



Jmie20 



Weight, average 

 (2), 26.8 g. 



1 yomig female 



Jmie20 



Weight 29.0 g. 



4 half-incubated 



Jimel 





eggs 







4 heavily Incu* 



Jmie 11 





bated eggs 







