ALTITUDINAL MIGRATION 107 



the north, while bands of snipe and duck, bred in 

 Arctic regions, swarm in the marshes. 



Similar movements occur in the Andes of Argen- 

 tina, where grebes and coots nest in mountain lakes 

 and winter in alkaline marshes on the plains, or in 

 the Rocky Mountains of our own country, where 

 the rosy finches, juncos, pine grosbeaks, and siskins 

 descend the mountain slopes in autumn and ascend 

 them in spring. In the Great Basin I have seen the 

 violet-green swallows leave their lowland haunts in 

 May for their breeding grounds in the hills, and 

 then during a late storm, which covered the moun- 

 tains with snow but in the valleys fell as cold rain, 

 return again to the lowlands. Such interchanges 

 meant only a few miles of flight and were accom- 

 plished with ease. 



In species of birds that inhabit or frequent both 

 hills and plains it has been noted in spring that first 

 arrivals may be seen near the foothills and from 

 there work upward. In autumn early migration may 

 be observed on the slopes of mountains or at their 

 bases, depending upon the species. There is, how- 

 ever, much variation in this, depending largely on 

 local conditions. In field work in the foothills of the 

 Front Range in Colorado, R. B. Rockwell and I 

 found that spring migration in the same species was 

 approximately a week later in the hills than on the 

 plains east of Denver, indicating apparently that 



