RAPIDITY OF MIGRATION MOVEMENT 113 



ern Canada. Here a part of the birds turn to the 

 northwest and move at an average rate of 50 miles 

 per day, and finally accelerate to a rush that carries 

 them across a space of 70 miles in twenty-four hours, 

 and at this increased speed arrive at their limit in 

 Alaska. Professor Cooke finds that their northward 

 movement coincides closely with the passage of the 

 isotherm of 35 degrees Fahrenheit, which brings 

 spring throughout the country. The differences 

 noted in average speed through our continent are 

 thus correlated with the average advance of spring, 

 so that the robin merits its reputation as the herald 

 who announces the close of winter. 



In connection with rapidity of migration it may 

 be well to note that there is often a distinction be- 

 tween the rapidity of movement in the bulk of the 

 species as a whole and of single individuals. At the 

 opening of migration a few birds of a species may 

 appear, but it may be a week or a month before the 

 bulk begins movement and the bird becomes com- 

 mon. Thus a few blackbirds reach our marshes early 

 in spring, but the return of cold weather may retard 

 the flights of their fellows for some time. Practically 

 nothing is known as yet of the manner in which 

 single birds travel, since our observations to date 

 have been restricted mainly to group identification. 

 Early stragglers form the vanguard in most of our 

 birds. It is apparent thus in many cases that cou- 



