ON NORTH AMiiRICAN ZOOLOGY. 187 



in a very agreeable and popular style ; but there is a want of 

 precise numerical data, which we trust Mr. Audubon's forth- 

 coming volume will amply supply ; in the mean time the follow- 

 ing pages, containing the chief statements made in the works 

 referred to, will give some idea of the question as it now stands. 



The primary objectof the migration of birds is generally allowed 

 to be the obtaining a due supply of proper food in the various 

 seasons of the year ; and it is to be observed that in many 

 cases the parents at the epoch of reproduction, and their callow 

 young, require a very ditferent kind of nourishment from that 

 which the species subsists upon at other times ; thus many, if 

 not most of the hard-billed granivorous birds, feed their un- 

 fledged brood on soft insects and grubs. 



Three lines of route, marked out by the physical features of 

 the land, are pursued by the bands of migrating birds in their 

 course through North America ; some species retiring on the 

 approach of winter through the eastern states and the peninsula 

 of Florida to the West Indies ; others passing down the great 

 valley of the Mississippi to the Texas and eastern Mexico ; and 

 others again keeping to the westward of the Rocky Mountains, 

 and entering the tropical regions by the shores of the Pacific. 

 Some more widely-diffused species pursue all the three routes ; 

 while others, hitherto detected only in a single tract in the 

 southerly part of their journey, spread from one side of the con- 

 tinent to the other as they approach their breeding quarters on 

 the confines of the arctic circle. Many birds, and more espe- 

 cially the soft-billed waders, make their flight northwards in the 

 higher latitudes through a different zone of country from that 

 which they traverse on their return southwards, being influenced 

 in this matter by the different conditions of the surface in 

 spring and fall. 



The short duration of summer within the arctic circle, taken 

 in connexion with the time necessary to complete the process 

 of incubation, the growth of plumage, and, in the case of the 

 anatidcB, the moulting of the parent birds, serves to limit the 

 northern range of the feathered tribes. The waders, which seldom 

 make a nest, and the water-birds, which lay their eggs among 

 their own down, and obtain their food on the sea or open lakes 

 when the land is covered with snow, breed farthest north. 

 The ptarmigans, which bi-eed in very high latitudes, and moult 

 during the season of re-production, migrate only for a short dis- 

 tance, and by easy flights ; and, their food moreover being the 

 buds or tips of willows and dwarf birch, can be obtained amidst 

 the snow. When we consider that at the northern extremity of 

 the American continent, and on the islands beyond it, the sum- 



