ZONES OF THE BOREAL REGION 29 



tially similar faunas. Consequently, Ptarmigan, Gyrfalcons, Snowy 

 Owls, Snow Buntings, and numerous species of water birds are found 

 in northern Eurasia as well as in northern North America. In fact, 

 as Allen ('93) has shown, 60 of the 65 genera of birds occurring in the 

 American Arctic are circumpolar. The parts of the Rocky Mountains 

 and Sierras reaching above timberline, where, at the border of perpetual 

 snow, Leucostictes, the Pipit and White-tailed Ptarmigan nest, 

 should, it seems, also be included in the Arctic zone, altitude rather than 

 latitude here giving the required low temperature. 



The Hudsonian zone marks the northern limit of forest growth of 

 firs and spruces. It will be observed that on both the Atlantic and 

 Pacific coasts, as well as along the Mackenzie River, its northern 

 limits are considerably extended; while southward it pushes a spur 

 down the crest of the Rocky Mountains with outlying 'islands' as far 

 south as Colorado and New Mexico. The Rough-legged Hawk, Great 

 Gray Owl, Pine Grosbeak, Northern Shrike and Alice's Thrush are 

 characteristic birds of this zone. 



The Canadian zone is distinguished by the high development of 

 its coniferous forests. Its extension southward along the Alleghanies 

 will be noted, its altitude increasing as the latitude decreases. Thus, 

 where primeval coniferous forests have not been destroyed, the Cana- 

 dian zone appears in Massachusetts at an elevation of 1,800 feet (Howe 

 and Allen), in Pennsylvania at 2,000 feet (Dwight), and in North 

 Carolina at 4,500 feet (Brewster). 



Reference to the Biological Survey map will show how numerous 

 are the Canadian zone 'islands' on the higher portions of our western 

 mountain systems. Evidences of this zone should also be shown at 

 least as far south as the southern end of the Mexican tableland, 

 where, at an elevation of from 8,000 to 13,000 feet in heavy forests 

 of pine and spruce, such characteristic Canadian species as the 

 Red Crossbill, Evening Grosbeak, Junco, Siskin, and Brown Creeper 

 are represented by closely allied forms which breed there in 

 abundance. 



In addition to the species just named, the Canadian zone is char- 

 acterized by the presence in the nesting season of the Spruce Partridge, 

 Hawk Owl, Goshawk, Three-toed Woodpeckers, Yellow-bellied Fly- 

 catcher, Canada Jay, White-throated Sparrow, Tennessee, Myrtle, 

 Blackpoll, Bay-breasted, Blackburnian, Magnolia, and Canadian War- 

 blers, Winter Wren, Red-breasted Nuthatch, Ruby- and Golden-crowned 

 Kinglets, Bicknell's, Olive-backed, and Hermit Thrushes. 



The Zones of the Austral Region. The Transition, Upper Austral, 

 and Lower Austral zones, as has been before remarked, are transconti- 

 nental, but differences in rainfall separate them into eastern humid, 

 western arid, and Pacific coast humid divisions. 



The eastern humid and western arid divisions merge into one 

 another at about the one-hundredth meridian, or, approximately, 

 where, in going westward, the prairies pass into the plains. To the. 



