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). * * * * * CHAPTER VIII THE PERILS OF MIGRATION The dangers to which migratory birds are subjected during their journeys are but little less than those which would befall them if they remained in unsuitable zones. During long oversea passages fatigue and hunger weed out the weaklings, sudden storms and adverse winds strike them where no land is near, and they are carried often far from the goal they aimed at. Predatory birds accompany them, taking toll _en route_, and predatory man waits for the tired wanderers with gun and net. Shore birds may rest upon the waves; sandpipers have been seen feeding as they walked upon the drifting weed of the Sargasso Sea, and steamers and other vessels frequently provide a rest for weary birds; but what happens to the many which find no haven? "Woe to the luckless warbler whose feathers once become water-soaked!--a grave in the ocean or a burial in the sand of the beach is the inevitable result," says Mr Cooke. A storm on Lake Michigan during spring migration piled many birds along the shore, and in the wider Gulf of Mexico many hundreds of passage birds were seen to fall into the water when caught, but 30 miles from land, by a violent "norther." Other similar sudden disasters have been recorded off our British coasts, even so far back as 1786, when, as quoted by Southwell, a Newcastle collier passed through water off the Suffolk shores black with vast numbers of drowned woodcocks. During normal migration birds may be brought to a lower elevation by strong contrary winds, or they may be bewildered by fogs and cloud and dropped nearer the surface; it is then that the travellers meet with disaster at our coastwise lights.