ALTITUDINAL MIGRATION 109 



In some cases we find altitudinal migration affect- 

 ing birds of limited range, which nest in the higher 

 zones of mountains and winter near their bases, or 

 perhaps pass farther south. With some species, as 

 the pipit of this country, a suitable breeding range 

 for some is found in the Arctic-Alpine zone of the 

 Rocky Mountains, from Colorado northward, while 

 others seek a similar zone in the tundras of Alaska or 

 British America. 



After the breeding season some species of birds 

 regularly work upward, to spend the late summer on 

 the higher slopes of the mountains. Such move- 

 ment has been recorded in the western house wren, 

 and has been observed in western kingbirds and in 

 other species. In the foothills of the California 

 mountains, where the late summer and early autumn 

 are extremely dry, T. I. Storer ^ has recorded that 

 such supposed permanent residents as the spurred 

 towhee, wren-tit, bush-tit, and Vigor's wren, regu- 

 larly perform an altitudinal migration, as in late 

 summer, after the breeding season, they move up 

 the slopes into the higher zones of the mountains, 

 where they remain until the coming of autumn rains 

 breaks the drought at the lower levels and so makes 

 conditions of life there easier. A fascinating account 

 of altitudinal migration remains to be written when 

 the late summer movements of hummingbirds in the 



^ Univ. Calif. Publ. Zool. (1925) xxvii, 3. 



