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 
