324 POPULAR SCIENCE MONTHLY. 



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 un- 

 doubtedly 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 gen- 

 erally accepted understanding of what migration covers. In the popu- 

 lar 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, snow- 

 flakes, 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 nar- 



