MIGRATION AND WEATHER 103 



when the temperature is still about 34. The 

 isotherm, however, has reached central Athabasca, 

 and the Mackenzie Valley and Alaska robins double 

 and quadruple their daily average on the north-west 

 diagonal to keep pace with the spring (19, 20, 21). 



Instances worked out in America and elsewhere 

 might be quoted to show how some species forge 

 ahead and others lag behind the vernal wave. Each 

 species needs separate tracing in its routes and times 

 and habits, but on the whole the movements have 

 relation to the changes in seasonal temperature. In 

 autumn the journey varies according to the time of 

 starting. Early fall migrants, and indeed the 

 majority of autumn migrants all the world over, 

 travel more slowly than in spring ; they are neither 

 impelled by sex-irnpulses nor the need to escape from 

 failing food supplies. A little later the supply does 

 slacken and with it the temperature cools, and if, 

 the changes are sudden southward migration is 

 accelerated. Migration, however, is such an ad- 

 vantageous and well-established habit that it usually 

 begins before hurry is necessary, and the birds loiter 

 southward, feeding as they go. 



Mr Cooke shows that in spring, weather seldom in- 

 fluences the start from the winter home, but the 

 average weather conditions regulate the average rate 

 of northward advance and the date of arrival at 

 the breeding home (22). 



