Page:Popular Science Monthly Volume 60.djvu/332

324 and striking of recent results for the purpose of showing that progress, although slow, is actually being made, and also in the hope that it may lead to further observation and record, for which there is undoubtedly abundant need.

In the first place, it may be well to define briefly certain phases of bird movement that are often overlooked or confounded with the generally accepted understanding of what migration covers. In the popular mind, and, it may be added, this is the correct view, a migratory species is one that regularly resorts to a given locality for the purpose of rearing its young, after which both old and young retire to some other, often widely different locality, where they pass the time before the next breeding season. In all temperate countries the migratory birds may be separated along these lines into two classes: first, those which come in spring, spend the summer and retire towards autumn; and second, those which pass through in spring to a breeding ground nearer the pole, and in the fall while on their journey south. The distinction between these two classes is obviously one of degree rather than kind.

The birds that come to us only in winter, such as Juncos, snowflakes, redpolls and Lapland longspurs, are not usually thought of as migrants, yet it requires but a moment's reflection to show that they are strictly so, and this leads to the general proposition that most birds throughout the world are constantly changing their location, but, as the individual is merged in the species, it is often difficult to obtain exact data on the subject. Because we see individuals of a certain species constantly about us, we call that a resident species, but, as a matter of fact, it is more than likely that not the same individuals are continuously under observation.

There is also another class known as occasional visitors, as the pine grosbeak and snowy owl, which may be absent for years, then of a sudden appear in great numbers. Their coming is supposed to be the result of a deficient food supply in their natural habitat far to the north, but the evidence for this is theoretical rather than actual. Hardly to be distinguished from these occasional visitants are the sudden incursions of species in a locality in which they have never been before known, as when a vast horde of nutcrackers spread over all Europe in 1844, or the erratic sand grouse, a bird of Central Asia, which has penetrated to England. But the climax of this restless and roving tendency in birds is reached in the stragglers that now and then are found hundreds, even thousands, of miles away from their homes, as when the Old World skylark is found in Greenland and the Bermudas, the American black-billed cuckoo in Italy, and our catbird and brown thrasher in Europe. While it may not be quite logical to class all these bird movements under the head of migration, as