Young robins quarreling at their bath. Photographed in the yard of Mrs. Granville Pike, North Yakima, Washington
There is a theory held by many naturalists that the migrating instinct dates back to the glacial period. According to this theory North America was inhabited originally by non-migrating birds. Then the great Arctic ice-cap began to move southward and the birds were forced to flee before it or starve. Now and then during the subsequent period the ice receded and the birds returned, only to be driven again before the next onrush of the Ice King. Thus during these centuries of alternate advance and retreat of the continental glacier, the birds acquired a habit, which later became an instinct, of retreating southward upon the approach of cold weather and coming back again when the ice and snow showed indications of passing away.
The Gathering Flocks.—To the bird student there is keen delight in watching for the first spring arrivals and noting their departure with the dying year. It is usually in August that we first observe an unwonted restlessness on the part of our birds which tells us that they have begun to hear the call of the South. The Blackbirds assemble in flocks and drift aimlessly about the fields. Every evening for weeks they will collect a chattering multitude in the trees of some lawn, or in those skirting a village street, and there at times cause great annoyance to their human neighbours.
Across the Hudson River from New York, in the Hackensack marshes, behind the Palisades, clouds of Swallows collect in the late summer evenings, and for many days one may see them from the car windows as they glide through the upper air or swarm to roost among the rushes. These Swallows and the Blackbirds are getting together before starting on their fall migration.
In Greensboro, North Carolina, there is a small grove of trees clustered about the courthouse which is a very busy place during the nights of summer. Here, before the first of July, Purple Martins begin to collect of an evening. In companies of hundreds and thousands, they whirl about over the tops of the houses, alight in the trees, and then almost immediately dash upward and away again. Not till dark do they finally settle to roost. Until late at night a great chorus of voices may be heard among the branches. The multitude increases daily for six or eight weeks, additions, in the form of new family groups, constantly augmenting their numbers. Some time in September the migration call reaches the Martins, and, yielding to its spell, they at once depart toward their winter home in tropical South America.
The Usual Movement.—Many of our smaller birds, such as Warblers and Vireos, do not possess a strong flocking instinct, but, nevertheless, they may be seen associated in numbers during the season of the northern and southern movements. Such birds migrate chiefly at night and have been observed through telescopes at high altitudes. Such observations are made by pointing the telescope at the disk of the full moon on clear nights. On cloudy or foggy nights the birds fly lower, as may be known by the clearer sounds of their calls as they pass over; at times one may even hear the flutter of their wings. There is a good reason for their travelling at this time, as they need the daylight for gathering food.
There appear to be certain popular pathways of migration along which many, though by no means all, of the aerial voyageurs wing their way. As to the distribution of these avian highways, we know at least that the coastlines of the continents are favourite routes. Longfellow, in the valley of the Charles, lived beneath one of these arteries of migration, and on still autumn nights often listened to the voices of the migrating hosts, "falling dreamily through the sky."
A small number of the species migrate by day; among these are the Hawks, Swallows, Ducks, and Geese. The last two groups also travel by night. The rate at which they proceed on their journey is not as great as was formerly supposed. From twenty to thirty miles an hour is the speed generally taken, and perhaps fifty miles an hour is the greatest rapidity attained. Flights are usually not long sustained, a hundred and fifty miles a day being above the average. Individuals will at times pause and remain for a few days in a favourable locality before proceeding farther. When large bodies of water are encountered longer flights are of course necessary, for land birds cannot rest on the water as their feathers would soon become water-soaked and drowning would result. Multitudes of small birds, including even the little Ruby-throated Hummingbird, annually cross the Gulf of Mexico at a single flight. This necessitates a continuous journey of from five hundred to seven hundred miles. Some North American birds migrate southward only a few hundred miles to pass the winter, while many others go from Canada and the United States to Mexico, Central and South America.