34 The Migrations of Birds 



In the first place, it may be well to define briefly certain phases of bird move- 

 ment that are often overlooked or confounded with the generally accepted under- 

 standing 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, Red- 

 polls, 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 con- 

 stantly 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 b<* quite logical 

 to class all these bird movements under the head of migration, as narrowly defined, 

 they are more or less clearly manifestations of the same influences and go to make 

 up the sum total of this wonderful ebb and flow of bird life. 



The origin, or perhaps better the origins, of this habit or instinct of bird 

 migration is exceedingly obscure. Many theories have been advanced to account 

 for it, but perhaps none has yet been offered that explains satisfactorily all its 

 multitudinous phases. For instance, it has been suggested that migration is the 

 result of the development or acquirement of the power of flight. That flight 

 has had much to do in making long extended migrations easily possible no one 

 can deny, but that it has been the cause is not logically evident, for certain 

 mammals, as the bison and antelope, are to a limited extent migratory, and cer- 



