° 1915 J Cooke, Bird Migration in the Mackenzie Valley. 455 



that range from the Rocky Mountains to the Pacific are Spizella 

 passerina arizonoe, Vireosylva gilva swainsoni, and Piranga ludovici- 

 ana. The migration route of this last species is especially interest- 

 ing. It breeds over the whole of western United States from the 

 eastern foothills of the Rocky Mountains to the Pacific. Hence if 

 one saw the map of the breeding range and noted that the line of 

 the easternmost limit was almost north and south and extended 

 without a break from Mexico to Canada, he would take it for 

 granted that the breeding birds of Alberta reached their summer 

 home by a migration route along the eastern slope of the Rocky 

 Mountains. Such reasoning is correct with almost all species, but 

 an examination of the large amount of migration data available 

 shows that the Western Tanager is an exception to the rule. The 

 bird winters in Guatemala and when it starts north in the spring 

 the individuals along the Pacific coast move north faster than 

 those that choose to migrate along the eastern slope of the Rocky 

 Mountains as shown by the isochronal lines on the accompanying 

 map. By May 10, the earliest migrants have reached northern 

 Washington along the Pacific coast, while in the Rocky Mountains 

 they are just entering southern Colorado. During the next ten 

 days the eastern birds loiter across Colorado to southeastern 

 Wyoming, while on the same date, May 20, the first birds appear 

 in central Alberta, a thousand miles farther north. It is evident 

 that these latter birds could not have come by way of Colorado, 

 but must have come from Washington and British Columbia, 

 though this latter assumption requires that they cross the main 

 chain of the Rocky Mountains at a time in the spring when even 

 the lowest passes are still covered with snow. It is true that warm 

 weather has already come by this date in the southern Mackenzie 

 Valley, but it is one of the strangest problems in bird migration 

 as to how the Western Tanagers know that on the other side of 

 those snow clad ranges summer is waiting for them. 



The migration dates of the Western Chipping Sparrow show that 

 the breeding birds of Alberta follow the same general route as out- 

 lined above for the Western Tanager, while the data so far available 

 concerning the migration of the Western Warbling Vireo throw no 

 light as to the route employed. 



The fifteen western species breeding in the Mackenzie Valley and 



