BLACK-POLLED WARBLER 391 



northern tier of Central States until about May 15. The individuals 

 that cross the upper Mississippi Valley through Minnesota and the 

 Dakotas then greatly accelerate their pace to about 200 miles per day 

 so that a week later they have reached the central part of the Mac- 

 kenzie Valley and by May 30 they arrive at the far distant breeding 

 grounds in northwestern Alaska. This long distance is covered in 

 about the same time that the slower-moving eastern contingent arrives 

 in southern Quebec and Newfoundland. About 25 to 30 days are 

 required to travel the distance of 1,000 miles from Florida to Minne- 

 sota, whereas the final lap of 2,500 miles is accomplished in less than 

 two weeks. This remarkable change in speed can be correlated with 

 the fact that the advance of spring in the northern interior is much 

 more rapid than it is in the Mississippi Valley and Gulf coast. In the 

 northland spring comes with a rush, and during the height of migra- 

 tion the temperature of the Mackenzie Valley is about the same as it 

 is in Minnesota. 



The black-polled warbler breeds principally in Canada and none 

 farther south than southern New Brunswick, the mountains of the 

 northeastern states and in the Rocky Mountains. This means that no 

 blackpoll migrates a distance of less than 2,500 miles, while those indi- 

 viduals that winter in Brazil and nest in northwestern Alaska must 

 travel more than 5,000 miles to reach their nesting grounds. These 

 are straight-line distances, and do not take into account the deviations 

 in the course or the random flights in search for food. 



The migration route of the blackpoll from Alaska has been evolved 

 over a long period of time. It is apparent that this bird extended its 

 range from the eastern part of North America to northwestern Alaska. 

 Instead of using the Pacific fly way it retraces its journey across the 

 mountains and moves southeastward over the ancestral route to leave 

 the United States for South America through a funnellike exit by way 

 of Florida. In Arkansas, Louisiana, and the southern part of the 

 Mississippi flyway it is a comparatively rare bird. 



In autumn tlie blackpoll begins leaving its outpost in Alaska in 

 August and, as in spring, it moves more rajDidly over the northern 

 section of its journey. After it reaches the States early in Septem- 

 ber its speed is less pronounced. These warblers are among the last 

 to leave ; they may be seen in the United States throughout September 

 and October, and the last individuals do not leave Florida until 

 November. 



The distribution of this warbler is not uniform over the entire area 

 included in the migration, and the details of its course taken in spring 

 may vary from that followed in autumn. T. D. Burleigh (1934), in 

 a study of the distribution and abundance of the black-polled and bay- 

 breasted warblers in the southeastern States during the spring and 



