MIGRATION OF BIRDS. 5 



intensity of color in regions of greatest rainfall. They are thus evi- 

 dences of an evolution so recent that we can assign its cause. 



In nomenclature these partly formed species are termed subspecies 

 or races. Under the trinomial system they are distinguished by a 

 third name : thus Colinus virginianus floridanus is a subspecies or race 

 of Colinus virginianus. But if through any change in the earth's sur- 

 face the regions occupied by the large, light Quail and the small, dark 

 one should be separated, the intermediates would disappear and in- 

 stead of a race or subspecies we would have a full species Colinus 



The Migration of Birds. Certain insects, fishes, and mammals 

 migrate, but no animals compare with birds in the extent of their 

 migrations. Some birds nest within the Arctic Circle and winter in 

 the southern parts of the southern hemisphere, performing this jour- 

 ney of thousands of miles year after year and returning to their sum- 

 mer or winter haunts with marvelous certainty and regularity. 



Not only is the migration of birds an interesting subject in itself, 

 but the almost constant changes it causes in the bird-life of the same 

 region gives an interest to the study of ornithology which is ever re- 

 newed with the changing seasons. Indeed, to the lover of Nature 

 birds are a living calendar. " What was that sound that came on the 

 softened air? It was the warble of the Bluebird from the scraggy 

 orchard yonder. When this^is heard, then has spring arrived." 



A discussion of the origin of bird migration will be found in the 

 books and papers on this subject to which reference has been given. 

 Here it is possible only to outline its principal features as they exist 

 to-day. Generally speaking, birds migrate at the approach of winter, 

 in search of food. The reason why they leave a land of plenty in the 

 tropics and follow the footsteps of retreating winter to nest in the 

 comparatively barren north is as yet not satisfactorily explained. 



As a rule, the extent of a bird's migration depends upon the nature 

 of its food. Insect-eating birds are forced to extend their migrations 

 much farther than the seed-eaters, many of which are permanent 

 residents at their place of birth. The land-birds of the Western 

 States winter in Mexico. A few cross the Isthmus of Tehuantepec 

 into Central America, but practically no species cross the Isthmus of 

 Panama. Our Eastern birds have a longer and more perilous journey 

 to perform. They leave the mainland by way of Florida, and some 

 species find congenial winter haunts in the West Indies or Central 

 America, while others go as far south as the Argentine Republic. 



Birds of strong flight, like Swallows, can easily escape from bird- 

 killing Hawks, and so migrate boldly by day. But the shy, retiring 

 inhabitants of woods and thickets await the coming of darkness, and 



