GEOGRAPHICAL DISTRIBUTION 
9 
Other conditions affect the distribution of bird life. The treeless 
prairies, with their dry uplands interspersed with myriads of shallow lakes 
and sloughs, fed largely by the melting of the winter's snow and in many 
cases strongly alkaline, attract an entirely different class of birds than do 
the heavily forested woodland to the north or east, or the mountainous 
country farther west. Another cause of differentiation in bird life is the 
barrier formed by the great western mountain ranges, which permits 
free range up and down the continent, but limits it east and west. Many 
species cross these mountains without difficulty, but others find them an 
almost complete barrier. Their effect has been to populate the regions 
west of the Rocky Mountains with many species that have never found 
their way east. These mountains have also broken up the country into 
numerous more or less isolated communities, with their own peculiar 
physical characteristics that have developed a large number of geographical 
or subspecific races. In consequence many species that show homogeneous 
characters from the Atlantic to the mountains break up into a number of 
special forms from the mountains westward. 
Taking the eastern forms as typical in the ordinary acceptance of the 
word, comparable birds of the prairie are slightly smaller and considerably 
paler in coloration, whereas on the humid Pacific coast they are larger and 
much darker. Through these influences we find in the west many sub- 
species of eastern forms. Comparatively few species range unmodified 
across the continent; many are represented east and west by two or more 
subspecies showing greater or less differentiation, and in other cases are 
locally replaced by closely allied species or are absent altogether. So far 
as birds are concerned these faunal divisions have to be based entirely 
upon breeding individuals. Birds travel so widely and along so many 
devious routes in their migration, that they may pass through several 
faunal areas in spring and autumn, though breeding in only one. There- 
fore, in determining the faunal zone to which any given area should be 
referred, transients must be disregarded. 
The following birds are representative of each life-zone. All of any 
one group may not be entirely confined to their zone, but in it they reach 
the centre of their breeding abundance and, associated together, they give 
the dominant characteristics of the bird life. Nor may all these species 
occur throughout the faunal zone to which they belong; for instance, some 
Transition species of the prairies do not extend across the mountains into 
the Transition of British Columbia, and vice versa. The lists are merely 
suggestive and might be greatly extended. 
Upper Austral — 
Sage Grouse 
Dickcissel 
Grasshopper Sparrow 
Chat 
Sage Thrasher 
Canyon Wren 
White-throated Swift 
Cardinal 
Orchard Oriole 
Blue-grey Gnatcatcher 
Transition 1 — 
Bobolink 
Baltimore or Bullock’s Orioles 
Eastern or Spotted Towhees 
Catbird 
Brown Thrasher 
Eastern and Western Bluebirds 
Ferruginous Rough-legged Hawk 
Sprague’s Pipit 
Chestnut-collared Longspur 
Wood Thrush 
Cuckoo 
Field Sparrow 
x Most of the species of this zone also occur in the Upper Austral, but reach their northern limit here. The 
occurrence of these, with the absence of the species mentioned as peculiar to the bordering zones, are the most 
marked characteristics of the Transition zone. 
