The Migration of Birds

by T. A. Coward

Map to show that a bird leaving Norway, near Aalsund, might be carried round the British Islands in twenty-four hours. The arrows indicate the actual directions and force of wind at the times marked during a slow-travelling circular storm in autumn 1901. Speed of bird about twenty-five miles per hour.

Even violent storms move at varying rates, and it is conceivable that a bird leaving Scandinavia on favourable anticyclonic winds might at once come into the influence of a large, slowly-moving, circular storm, with a low-pressure centre to the west of Ireland, and might, if the air currents were strong, be carried westward at first, then south and finally eastward, so that it would actually pass round the British Islands. I have taken this exceptional case from the actual course of a storm, which varied between forces 9 and II on the Beaufort Scale (say an average of 50 miles per hour) but only travelled slowly eastward at about 17 miles per hour. In some cases the storm centres are nearly stationary for many hours.

It is easy to appreciate Herr Herman's statement that spring immigration in Hungary is accelerated on the good side of a mild cyclone; the direction of the bird, of the circulating air currents and of the whole system may be coincident. Given a low-pressure centre west of the Bay of Biscay, spring migration would be accelerated through Spain and France towards Britain.

Mr Stubbs points out that the pathways of several birds, or parties of birds, which started at different hours, would be divergent, for they would come within the influence of winds blowing in various directions according to the position of the system; this he argues is contrary to the accepted idea of routes. This, however, entirely depends upon what we mean by a route, as I endeavoured to show in an earlier chapter. The journey from point to point is a route, although the bird may be drifted many miles in one direction or another on the way; it is only when the bird fails to reach its objective, a suitable breeding place or winter station, that the route is a failure.

The frequent occurrence of rare birds, some of them almost or quite unknown elsewhere in Britain, on out-of-the-way islands, has led to strange theories. One is that there are regular fly-lines over Fair Island, the Flannens, St Kilda and elsewhere, similar to the one which is said to pass over Heligoland. Mr Eagle Clarke's long expected book will contain the ideas of the man who is best able to theorise on this point; I write, now, with the feeling that his knowledge may lead me to alter my ideas. The suggestion I can offer at present is that there are ornithologists directing their attention to these spots which, through geographical position and isolation, are the likely refuges for wind-borne migrants. Also that the accidental departure from the directions aimed at by the birds is, where wind and barometric systems are so variable, far more frequent than is usually suspected. Direct routes are doubtless aimed at, but only accomplished under favourable conditions for the whole journey; migration is less infallible than we have been led to think. It is, too, an evolving habit, strengthened by those which survive its perils, now as it was in its early days.

During a long overland journey, winds will probably have less influence, though for rapid passages high flights certainly appear to be not uncommon. There is, however, another aspect of the connection between migration and weather which we have hardly touched, migration synchronal to the change of season. Mr Cooke shows that in North America the push forward in spring is not in most species so soon as the weather permits; they do not actually move on the spring wave. Many warblers which nest in the Great Slave Lake region in an average temperature of 47°, linger in the Tropics, and reach New Orleans when the temperature is about 65°F. Then they hasten northwards, outstripping the advancing spring, finding in Minnesota a temperature of about 55°, and 52° in Manitoba, and gain another 5° on the season by the time they reach their home. Thus they continually reach colder weather as they travel north.

The American robin, Turdus migratorius, moves more sedately; it takes seventy-eight days for its 3000 mile trip, whilst spring takes some ten days less to cover the distance. But the individual robins may advance more quickly; it is the robin as a species which takes this time to cover the area of distribution. The isotherm of 35°F., corresponding to the beginning of spring migration, advances north at the rate of 3 miles per day from January 15th to February 15th; 10 miles a day is the average for the next month, and 20 for the following month. But along the eastern foothills of the Rockies, isotherms travel faster than in corresponding latitudes farther east; spring rushes to this western land. In mid-April to mid-June—the height of migration—the southern portion of the Mackenzie Valley has about the same temperature as the region of Lake Superior 700 miles farther south. This, coupled with the diagonal course of the birds across the fast-moving region of spring, exerts a powerful influence upon migration; the earliest robins reach southern Iowa on March 1st, and travelling northward at about 13 miles per day, find in central Minnesota a temperature similar to the one they left. Those which breed near Lake Superior increase their speed to a daily average of 25 miles, and arrive at latitude 52°, 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)].