THE DISTRIBUTION OF ANIMALS 465 



the salmon migrate up the rivers and spawn in the very streams in which 

 they were hatched. Birds, of course, make the most extensive migra- 

 tions. Most of the birds of the United States are migratory ; although 

 a few, including the bobwhite, the ruffled grouse, and the cardinal, are 

 nonmigratory. The waterfowl, such as ducks, geese, and swans, are the 

 most conspicuous of the migratory birds. By systematic banding the 

 Fish and Wildlife Service had discovered that these birds follow four 

 special routes, known as the Atlantic flyway, Mississippi flyway, Great 

 Plains flyway, and the Pacific flyway. Many of our songbirds appear 

 to have been derived from South American birds and to have come into 

 the United States in the summer in search of a place to raise their young. 

 The wood warblers are typical of this group. As soon as they have 

 raised their young, they return to their ancestral home in South America 

 at a time when the climate of the United States is still warm and food is 

 abundant. Of forty-six species of wood warblers there is only one, the 

 myrtle warbler, which regularly winters north of the subtropical por- 

 tion of the United States. In contrast, the Old World warblers, repre- 

 sented in the United States by three species — the golden-crowned kinglet, 

 the ruby-crowned kinglet, and the blue-gray gnatcatcher — may winter 

 as far north as southern Canada. None of these three go as far as 

 South America. 



Some butterflies also have annual migrations. One of the best known 

 of these is the monarch or milkweed butterfly. In the fall of the year 

 they may be observed moving leisurely southward in great flocks. Un- 

 like most butterflies they do not hibernate, lay eggs, or form chrysalids 

 for over-wintering. The following spring they gradually move north- 

 ward with the season. 



Reduction in Range 



One has only to examine a fossiliferous rock to discover many 

 species of animals and plants which no longer occur in any particular 

 region. Mastodons, mammoths, and camels were formerly common in 

 the United States. Many animals are being reduced in their range 

 today because of the impact of expanding civilization. Many of our 

 game animals have been driven from all but a small part of their former 

 range. The American buffalo, or bison, is now limited to a few herds in 

 some of the western parks and a few specimens in various zoos of the 

 nation. The whooping crane is reduced to a flock of about twenty birds, 

 and the California Condor has become very scarce. The mighty stur- 

 geon of the northern rivers has been caught so extensively for its roe, 

 which is made into caviar, that it has become rare in most streams. 



