Vol 'm^ XIT ] Cooke, Bird Migration in the Mackenzie Valley. 445 



zie, not a mountain chain or even a ridge of hills interferes with the 

 uniform movements of the birds. The highest elevation is less 

 than two thousand feet, and so gradual are the slopes that, with a 

 few short portages the whole distance can be traversed in a canoe. 

 The whole region is well watered and well timbered, affording ideal 

 conditions for the support of the multitudes of birds which swarm 

 along this route as they do nowhere else on the North American 

 Continent. 



If the mouth of the Mackenzie River was due north of Louisiana 

 and in the middle of the continent, bird migration by this route 

 would be a uniform progression from south to north in the spring 

 and the reverse in the fall. On the contrary the valley of the 

 Mackenzie lies nearly two thousand miles nearer to the Pacific 

 than to the Atlantic Ocean, and the warm Japan current produces 

 conditions that interfere with the uniformity of migration and bring 

 about variations, probably not equalled anywhere else in the world, 

 both in the direction and the speed of migration. 



That this diagonal northwest and southeast route is traversed 

 by birds from the Mississippi Valley is shown positively in the case 

 of thirty-three species, for these breed in the Mackenzie Valley and 

 pass in migration across the United States and yet occur in the 

 United States as far west only as the eastern edge of the plains. 

 Hence it is certain that these thirty-three species have a northward 

 migration in the Mississippi Valley from eastern Kansas to western 

 Minnesota and thence a northwestward route to Lake Athabaska. 



This is shown on the accompanying map of the distribution of 

 the Rose-breasted Grosbeak (Zamclodia ludoviciana) . 



It is evident that the westernmost breeding birds, those that 

 summer in the Mackenzie Valley must have reached their breeding- 

 grounds from the southeast by way of the Mississippi Valley. 



The cause of the choice of this route is easily found in the con- 

 ditions of moisture and woodland. All these species are either 

 lovers of damp forests or of moist meadows and marshy lakes. 

 Their favorite surroundings extend in the United States not farther 

 west than eastern Kansas and western Minnesota. On arriving 

 at Manitoba, the dry plains that have been a barrier on their left 

 for the last thousand miles, become better watered and interspersed 

 with groves and soon these groves unite to form almost continuous 



