found. This generalization is in harmony with the geographical relations of the continent to other land areas, and with the fundamental principles of evolution.
Migrations.—Among the many facts of interest to the geographer in connection with the bird life, none present a more fascinating field for study than the annual migrations in which a very large number of the species participate.
As one travels northward from Mexico or the Gulf States, the number of species of birds which remain in essentially the same area throughout the year, or the residents as they are termed, becomes less and less. In New England and about the southern shores of the Great Lakes there are about 30 species which remain all winter and may justly claim to be citizens. Besides these, there are several visitors that come from the north and belong to the vast army of migrants, but which are contented with a comparatively small change of position during the periods of greatest cold or heaviest snow. In the far north the number of residents is still more restricted. On the tundras fringing the arctic coast even the snow-owls, snow-buntings, and the ptarmigans, the hardiest of birds, move southward during the winter to the shelter of the subarctic forest, and bird life on the vast frozen morasses is practically, if not absolutely, wanting.
The millions of birds that journey southward each fall begin their return migrations at the first promise of spring. Even during unusually mild spells of weather in winter, temporary northward movements occur. The migratory birds are actuated by such a strong desire to regain their nesting places and summer homes that they embrace every opportunity to journey towards them, and not infrequently suffer severely for the risks they take. In some instances species which have begun their northward flight too soon are killed by thousands owing to a return of severely cold weather or die for lack of food.
The first definite northward migration in the southern portion of the Mississippi Valley begins during exceptionally favourable years as early as the end of January, but
the great movement of the feathered hosts is not usually at its height before the middle of March or the first of April. In New England the current of migration begins between the middle of February and the first of March, and increases in strength until the middle of May, when it is at its height, and then rapidly declines and is practically over by the beginning of June. In the far north, the first arrival from the southward, and that a species which does not make a long annual journey, usually appears early in April. At Point Barrow, the most northern portion of Alaska, as was observed by John Murdock in 1882 and 1883, the first harbinger of spring was a snow-bunting, which arrived the first year on April 9th. The northward-flowing tide of bird life ends early in July in the region of the Yukon, and by the middle of that month the vast flocks have been separated and the many mates have found their nesting places. The time taken for the general movement is thus in the neighbourhood of four months.
The northward flight of the birds is seldom, if ever, one continuous journey, but like many other movements in nature, progresses by pulsations. Well-defined "bird-waves" have been recognised especially in the Mississippi Valley. The direct or immediate cause of the starting of these waves of life is the coming of a wave of heat. Secondary or modifying conditions are furnished by strength and direction of the wind, cloudiness, rain, etc. As the weather in spring-time is fickle, and its variations not the same for any two consecutive years, so the gathering of the birds into flocks and their northward flights vary, although for a term of years the arrival of a given species at a particular station does not depart far from a mean date. With the northward sweep of the waves of bird life over the temperate and boreal portions of the continent comes the awakening of plant life, but the birds, to a marked extent, precede the unfolding of the flowers. This marvellous renewal of the life of the land after the long cold winter makes the budding and nesting spring-time the most joyous portion of the year, and one which exerts a marked effect on human thought and activities. The
spring-time awakening in all nature is like a resurrection, and has apparently exerted an influence on the religions of the world.
The bird-waves referred to above are characterized at the start by the presence of great numbers of a single species, but as they progress, scattering occurs, and at the time of the greatest movements in the northern portion of the United States a large number of species frequently arrive at a given locality in a single night. At sunset the groves may be nearly tenantless and silent, while at sunrise they are alive with the flitting of wings of many colours, and the air pulsates with many different songs.
Judging from extended observations on migrations made in the Mississippi Valley, the definite waves of bird life which sweep northward with the spring-tide of temperature are in some instances 100 or 200 miles long and have a breadth of perhaps a score of miles. The distance between the waves varies with variations in the weather and perhaps other causes, and as they progress they apparently become less definite and at the north have yet to be recognised.