ARCTIC-ALPINE BIRD-LIFE OF THE ROCKY 

 MOUNTAINS 



WHERE the summits of the Rocky Mountains, Sierras and Cas- 

 cade Range reach above timber-line to the limits of perpetual 

 snow, the boreal nature of the climate produces conditions 

 favorable for the existence of many plants and animals which, in the 

 Arctic Regions, are found at sea-level. The altitude at which these 

 conditions appear increases as the latitude decreases. For example, 

 in Colorado, at latitude 40°, timber-line is at 11,000 feet altitude, while 

 in Alberta, Canada, at latitude 50°, timber-line is at about 7,500 feet. 



Where the area of sufficient altitude is practically continuous the 

 presence of arctic forms of life may be due to extension of range south- 

 ward; but where it is discontinuous, a boreal area may be separated 

 from a similar region to the north by intervening lower ground, when 

 the occurrence of boreal forms may be attributed to the influence of the 

 Glacial Period. Forced southward during the Ice Age, they were left 

 stranded on these high Arctic-Alpine islands as the ice receded. 



The characteristic Arctic- Alpine birds of our western mountains are 

 the White-tailed Ptarmigan, the Rosy Snow Finch (Leucosticte) and the 

 Pipit. In the Rockies they are found as far south as Colorado or New 

 Mexico. The Pipit migrates southward in winter, but the Ptarmigan 

 and Snow Finch are practically permanent residents, in winter descend- 

 ing only slightly below timber-line. At this season, as is well known, the 

 Ptarmigan acquires a pure white plumage. (For a seasonal group of 

 Ptarmigan and an explanation of their plumage changes, see the Main 

 Bird Hall.) 



The studies for this group were made in the Canadian Rockies 

 about fifteen miles north of Laggan at the Ptarmigan Lakes. The 

 party outfitted at Lake Louise, and ascended the mountains to the 

 northward, from which one obtains a beautiful and impressive view. 

 At the left, beyond Mount Redoubt, in the foreground, lie the peaks of 

 Moraine Lake; in the center is Mount Temple; and to the right, 

 Hungabee, Lefroy and Victoria. 



At this season (July 15, 1907) the alpine spring was at its height. 

 The lakes were opening; great white anemones were blooming at the 

 border of the snowfields; the heather was white with little bell-like 

 flowers, and the beds of fluted Dryas leaves were starred with blossoms. 



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