452 Cooke, Bird Migration in the Mackenzie Valley. [bet. 



made of the general principle of parallel lines of migration. Since 

 it is true that the birds of western Minnesota pass northwestward 

 to Saskatchewan, it is probable that the birds of Montana also 

 proceed in a northwestern direction and traverse Alberta, in which 

 case it is altogether unlikely that the birds of the same species would 

 be migrating northeast from British Columbia to Athabaska across 

 the route of the eastern birds and at right angles to it. A more 

 reasonable assumption is that the birds of British Columbia migrate 

 also northwestward and proceed to Alaska, where the dates of 

 arrival, May 5 at Fort Reliance on the middle Yukon and May 18 

 at the mouth of that river, show that the birds musl? have come 

 from British Columbia. 



The migration records now available are insufficient to determine 

 whether the remaining thirteen species of small migratory wide- 

 ranging land birds come to the Mackenzie Valley from the south- 

 east, or southwest. 



There remain twenty-three species of migratory birds which 

 breed in the Mackenzie Valley, but which in migration are confined 

 to the Western United States, ranging not farther east than the 

 eastern edge of the plains. These may be divided into three groups. 

 The first group comprises nine species that in winter are confined 

 to the Pacific coast; the second group, three species that range east 

 to the Rocky Mountains, and the third, sixteen species that occur 

 east to the plains. 



The species of the first group, whose routes are best known, are 

 the Pacific Eider {Somateria v-nigra), Black Brant {Brant a nigri- 

 cans), Short-billed Gull {Larus brachyrhynchus), Ross's Goose 

 {Chen rossi), and the Northern Varied Thrush {Lvorcus naevius 

 mcruloidcs). The accompanying chart shows the principal migra- 

 tion route followed by each of these species. They all cross the 

 Rocky Mountains, but in widely separated latitudes. The Ross's 

 Goose crosses the lofty ranges of the main chain of the Rock}' Moun- 

 tains from northeastern California to northwestern Montana and 

 thence north across the Mackenzie Valley to its breeding grounds 

 on the Arctic islands. 



The Northern Varied Thrush winters mainly in the interior of 

 California and in western Nevada. Its main migration route crosses 

 thence through northern Idaho and northwestern Montana and 



