324 U. S. NATIONAL MUSEUM BULLETIN 217 



There is such a sudden change in the activity of migratory birds 

 when they reach their nesting grounds that it would be surprising 

 if visible signs failed to indicate a change in their physiological con- 

 dition. The northward progress of migration involves all migrants 

 to Anaktuvuk in flights which must proceed at a rate sometimes 

 greater than 60 miles a day in order to conform with the indications 

 of the northbound schedule. After reaching their destination many 

 species do not fly often or far and become sedentary birds in contrast 

 with their activity during migration. The mechanisms of flight 

 remain usable but they are little exercised. 



There is a possibility that the musculature of flight may diminish 

 in migratory birds which become sedentary on their nesting grounds. 

 The evidence available is insufficient to suggest whether the birds like 

 gulls, jaegers and hawks, which continue to fly extensively while on 

 their breeding grounds, maintain their migratory weight while 

 breeding. 



The average weight of specimens of birds in various parts of their 

 range is known to differ in some cases without other visible taxonomic 

 distinctions appearing. Von Zedlitz (1924) reported that the average 

 weight of Tetrao urogallus from Sweden was 3.65 kg. and from 

 Hinterpommern 5 kg. I have discussed the variations in the weight 

 of willow ptarmigan {Lagopus lagofus alascensis) from various 

 localities, which appear to be significant of populations differing in 

 weight although without taxonomic distinction. It is not known 

 whether these differences in weight are maintained through heredity 

 or whether they are simply induced in individuals by the two environ- 

 ments. 



The birds nesting in the part of arctic Alaska we are considering 

 are geographically homogeneous populations, and this factor may be 

 important in the small "coefficients of variation" which were found in 

 the weights of the species. I have taken as "coefficient of variation" 

 the usual meaning, i. e., standard deviation as percent of the mean 

 weight. In the weights of adult males of 21 species the coefficients of 

 variation were between 5.3 and 12.6 percent. Among 9 species the 

 coefficients of variation in the weights of female birds were from 7.2 to 

 12.4 percent. The average of the coefficients of variation in male 

 examples of 21 species, each composed of from 10 to 40 individuals, 

 was 8 percent. Among samples of the females of Y species the average 

 of the coefficients of variation was 9 percent. Considering the chances 

 for error in gathering figures for weight under the conditions of 

 arctic field study the birds of each species appear homogeneous in 

 their weight characteristic at Anaktuvuk. 



The weight of a small bird may change considerably during the 

 course of a day, as is shown in the report by Linsdale and Sumner 



