BIRD MIGRATION. 23 



trend; but as soon as the coast line turns westward near the north- 

 western point of British Columbia the birds disappear and are not 

 known anywhere in the 500-mile strip between the Pacific coast and 

 the Mackenzie Valley. Apparently this region is crossed at a, single 

 flight from the salt water of the coast to the fresh-wahT summer 

 home on the great lakes of the Mackenzie Valley. 



A migration route entirely different from any thus far mentioned 

 is that of the western tanager, or Louisiana tanager, as it was for- 

 merly called. From its winter home in Guatemala (see fig. 9) it 

 enters the United States about April 20; another 10 days and the 

 van is in central New Mexico, Arizona, and southern California, 

 marking an approximately east and west line (see fig. 10). The 

 next 10 days the easternmost birds advance only to southern 

 Colorado, while the western have reached northern Washington. 

 May 10 finds the line of the van extending in a great curve from 

 Vancouver Island northeast to central Alberta and thence southeast 

 to northern Colorado. It is evident that the Alberta birds have not 

 reached their breeding grounds by way of the eastern slope of the 

 Rocky Mountains, a route which would naturally be taken for 

 granted by anyone examining a map of the winter and summer homes. 

 On the contrary, these Alberta breeders must have come by way 

 of the Pacific coast to southern British Columbia and then crossed 

 over the main range of the Rocky Mountains, which at this season 

 (May 20) are still cold. and partly covered with snow. 



Still another strange migration route, probably unique, is that of 

 the Ross snow goose (see fig. 11). This species breeds on the 

 Arctic islands north of Mackenzie and in fall migration it travels 

 up the valleys of the Mackenzie and Athabaska Rivers in company 

 with thousands of other waterfowl bound for their winter homes on 

 the coasts of eastern United States and the Gulf of Mexico. But 

 on reaching the northern boundary of the United States the Ross 

 goose parts company with its traveling companions, and while they 

 continue south and southeast along the usual migration route it 

 turns to the southwest, crosses the main range of the Rocky Moun- 

 tains, and settles for the winter in California. 



WIDE AND NARROW MIGRATION ROUTES. 



The shape of North America tends to a converging of the lines of 

 migration toward the Gulf of Mexico, and consequently the east and 

 west breadth of the migration route just south of the United States 

 is usually less than the corresponding breadth of the breeding terri- 

 tory. The extent to which migration routes contract varies greatly 

 with different species. The redstart represents one extreme (see fig. 

 12), where the lines of migration are carried far eastward to include 

 the Bahamas and the Antilles, while they also extend southwestward 



