788 POPULAR SCIENCE MONTHLY. 



that swallows hide through the winter in holes and in clefts in the 

 rocks, and even under the water. 



Many writers on migration have believed, as they have been 

 taught from childhood, that the birds go south to escape the rigors 

 of a northern winter, although little reflection is needed to show that 

 no animals are better protected or more indifferent to changes of 

 temperature, or that, while sea birds are highly migratory, the open 

 waters of arctic seas are little colder in winter than in summer. 

 Nestlings are often killed by exposure, and eggs require a high ex- 

 ternal temperature, but old birds are, as a rule, indifferent to cold. 



When this is recognized, the prevailing belief is that birds leave 

 their homes in search of food, for scarcity is most assuredly an im- 

 portant factor in the origin of migration ; but this view of the matter 

 fails to show why, with the whole world to choose from, they do not 

 settle in lands which are habitable the year round. 



" The shuddering tenant of the frigid zone 

 Boldly proclaims the happiest spot his own," 



and the return of the birds seems only natural to the Eskimos; 

 but to us who are not Eskimos the wonder is not that anything 

 which can get away should do so, but why the birds pass by so many 

 lovely and fertile regions to seek a home in the barren and desolate 

 ends of the earth; and it is plain that, of the two journeys which 

 make up the migration, the summer visit to northern lands and 

 waters is at least as remarkable and as well worthy of consideration 

 as the journey southward in the fall. 



Failure of food in their birthplace is no doubt the chief reason 

 why the migratory birds do not spend the whole year there, and 

 in so far is an explanation of migration, for no animals are better 

 fitted for moving from regions of scarcity to regions of abundance, 

 although they are no more able than creeping things to establish 

 themselves in new lands which are already well stocked with in- 

 habitants; for they are kept within the limits of their natural 

 habitat, like other animals, by competitors and enemies, rather than 

 by physical barriers, although their power to wander and to over- 

 come physical barriers is without a parallel. There are few oceanic 

 islands, however remote, which are not inhabited by land birds 

 descended from lost wanderers, who, finding these spots unoccupied, 

 have been able to establish themselves. The list of North American 

 birds which are occasionally found in Europe is a long one, and stray 

 specimens of the gray plover, whose summer home is the shore of 

 the Arctic Ocean, have been found at the Cape of Good Hope, in 

 Ceylon, in Australia, in New Zealand, and in Tasmania. Most of 

 the wanderers are shore birds which make long migrations, and be- 



