The migrations of the birds are performed principally at night. In the northern part of the United States during the hours of darkness in early spring, even when cloudiness prevails or the land is veiled in mist, the voices of geese may frequently be heard overhead, proving that unseen flocks are then winging their way northward. About the lighthouses along the coast and on the shores of the Great Lakes, when migration is in progress, dead birds are frequently found in considerable numbers and of various species. In these cases the birds are evidently attracted by the lights and killed on striking the windows that protect them. This occurs particularly on stormy or cloudy nights, when the birds fly low. Several of the larger species of birds, as the geese, ducks, cranes, etc., which are strong of wing, make long flights without resting. In many instances a single stage in a journey may include 500 or 600 miles. Most of the smaller birds, however, fly comparatively short distances between the pauses made for rest and food.
A species on reaching the northern portion of the route over which it usually migrates scatters, and the individuals mate, nests are built, and young reared. At the approach of cold weather reassemblage occurs, frequently great flocks being formed, and the southern movement begins. The southward migration is less conspicuous in most instances than the movement en masse of the birds in the spring, and so far as now seems to be recognised is not divided into definite waves.
While the winter habitat of most birds in the temperate and boreal portions of the continent is to the south of their summer homes, the annual migration is not in all cases great in amount. Some species move only a few hundred, or possibly a few score, miles. Even the winter residents make short migrations, dependent on weather conditions. The greater part of the migratory birds, however, pass the winter in the Gulf States, Mexico, the West Indies, and Central and South America. In some cases they go well to the south of the equator. The annual flight going and coming measured in a straight line, between the nesting place and the winter home, cannot be less in many instances than from 8,000 to 10,000 or 12,000 miles. An interesting fact in this connection is that certain species follow definite routes. The region moved over annually, if marked on a map, would resemble two open or partially opened fans, with their handles pointing towards each other and connected by a narrow band.
The causes of the annual migrations of birds have received much study and been the subject of much speculation. The general consensus of opinion in this connection seems to be that the birds are controlled largely by what we in our ignorance term instinct. The true beginning of the migration seems to be in the fall, when the birds are driven from their homes by cold or, perhaps more accurately in most cases, by scarcity of food. This, however, is not the whole story, since many species start southward before cold weather approaches and while food is yet abundant. Then, too, crippled individuals have been known to survive the winter in regions from which their summer
companions have departed. Instinct, therefore, plays a part in even the fall migration, where at first glance sufficient physical reasons may seemingly be claimed for it. During the spring migration the birds are moved by a strong impulse to regain their breeding-grounds. Each species seems to have adapted itself to certain conditions of temperature, food, etc., through long ages of development, and acquired a subtle faculty of regaining the environment to which it is best adapted, as soon as the adverse conditions that caused it to leave its home are ameliorated. How a particular bird is enabled to return to the nest it built the year previous is not known. The study of the homing instinct of pigeons assists in this direction, however, and suggests that birds are endowed with something answering to a sixth sense—that is, a sense of direction or of orientation.
Spring-time Music.—The northward-flowing tide of life each spring brings to the temperate zone of North America a marvellous change not only in colour and movements, but in sound. This is the season of bird courtship and more than usual happiness among the feathered millions. From shore to shore of the continent a chorus more seductive than sirens' songs pulsates on the breezes.
The winter is characteristically a season of silence. The sounds heard at a distance from human habitations are mainly those produced by inanimate nature. The wind causes varied discords amid the bare branches of the deciduous trees or sings weird melodies in the pines. Strange muffled roars come from the frozen lakes, as the ice contracts and breaks during periods of excessive cold. The frost in tree trunks causes sharp explosions. The ice-covered streams are still except where cataracts interrupt their even flow. In the profound silence of a calm winter night the distant dismal howl of a wolf, the cry of an owl, or the bark of a fox alone reminds one that life still continues, but these animate sounds are far more frequently absent than present. With the coming of the spring there is a marvellous awakening and unfolding. The brooks, swollen to overflowing by the melting of the
snow, make music as they run. The northward flight of the birds brings to every grove a chorus of song. A host of batrachians and reptiles bestir themselves after a long winter sleep and vociferously proclaim their presence. The insect world, with its unnumbered legions, takes wing. The air vibrates with millions of voices. The trees put forth their leaves, each a harp-string which responds to the touch of the fingers of the wind. The organ-notes of the thunder again startle the hibernating echoes. As the winter is the silent season, so the spring is the time of music.
One of the most charming of the many phases of nature's concert season is the matin songs of the birds. Ere the eastern sky along the New England coast becomes roseate with the first blush of morn, the twitter of birds may be heard amid the shadowy branches of the trees. Soon a thrush or a warbler awakens in full song, and is followed by a host of other voices, until the air pulsates with music. As the sun rises and his first level rays reveal the varied tints of the tree tops, the many-voiced chorus passes the height of its ecstasy and the music gradually subsides. But the glad tidings of the coming of the day are passed westward from grove to grove and from meadow to meadow, and a wave of song sweeps on ahead of the wave of light, induced by its coming. The song-wave spreads to the north and south and flows steadily westward over the forest-covered mountains, across the great central basin of the continent, breaking on the treeless plateaus into many streams which follow the grove-fringed rivers, passes through the depressions in the Rocky Mountains, and although weakened in the arid valleys beyond, is not checked. The larks there listen for its coming and pass the joyful message westward. The timid dwellers in the great forests of Oregon awaken at the magic sound and the lofty tree tops are made to thrill with the voices of unseen choirs while it is yet night in the silent aisles below. The onward rush of sound is not reflected or turned back by the lofty Cascades, but flows through their passes and only ceases when the sea-birds of the