Page:The Migration of Birds - Thomas A Coward - 1912.pdf/127

Rh 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-impulses 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 advantageous 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 influences 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).