The migratory movement of the North American birds is almost wholly limited to the Atlantic coast, a smaller number being permanent residents than on the Pacific coast, or in corresponding European localities. In Massachusetts the regular number of summer visitors is 106, while there are only 30 species which remain all the year. The number of permanent residents increases as we go southward, but during the breeding-season in any single locality it increases as we go northward until we reach Canada, where more species rear their young than in the Southern States. The extent of the migration of certain birds has greatly altered within a limited period of observation. A Mexican swallow (Hirundo lunifrons) first appeared in Ohio in 1815; its yearly range increased, until in 1845 it had reached Maine and Canada, and now its annual migrations extend to Hudson's Bay. The rice-bird, or "bobolink," enters the Southern States in April, passes northward until in June it reaches Canada, and stops in its westerly course at the Saskatchewan River, in 54° north latitude, having widened its range continually as wheat and rice were cultivated over more extensive areas.
A nocturnal concourse of birds sometimes occurs in the neighborhood of large towns near the end of summer, in still, cloudy weather. The notes of well-known birds may be recognized by the skillful ornithologist, at one time faint in the distance, at another near by, while occasionally the stroke of a wing gives a sense of nearness to these remarkable visitors. It is supposed that these noises proceed from migratory birds which, having lost their way, are attracted by the light from street-lamps.
It is thus obvious that the migration of birds is no mere arbitrary matter, but is governed by laws susceptible of intelligent interpretation. Want of food is the most evident cause of their journeyings. As it becomes scarce near the end of summer in the extreme northern limits, those individuals which feel the pressure of want seek it elsewhere, and, in doing so, they press upon the haunts of other birds, until the movement which began in the north has extended to the southern limit. The power of flight in birds makes it possible for them to cross a moderate breadth of sea and unlimited extent of country, and, traveling as they do, mostly at night and high in the air, their movements seem mysterious, simply because they are difficult to observe. But, let us map their comings and goings faithfully as we may, there yet remains the unanswered question, How do these little visitants find their way so unerringly from one place to another, over great distances and apparently unexplored routes?
Some of the largest Mammalia are not stopped by any physical obstacle in their journeys over whole continents. The rhinoceros, the lion, and the tiger, have great powers of dispersal, and their possible range is unlimited wherever there are land and sufficient food. The elephant climbs to mountain-tops, difficult of ascent for man, crosses