296 BIRDS 



southward as winter comes on. Most of the birds from the 

 central and northern United States go to Mexico and Cen- 

 tral America in the autumn, to remain during the winter, 

 returning again in the spring. The extent of the migra- 

 tions of different birds varies greatly. Some go to Alaska 

 or the region of Hudson Bay to rear their young, returning 

 in autumn to Mexico and the West Indies. Others go no 

 farther than the Great Lakes, while others, known as 

 resident birds, remain throughout the year in the same 

 localities in which they are bred and reared. 



It is probable that the original cause of these migrations 

 was a change in chmate whereby there came about a 

 scarcity of food. The birds were thus forced to journey 

 to other regions in search of something to sustain Ufe. 

 The wandering, thus induced by force, has now been regu- 

 larly practiced by so many generations that it has become 

 a fixed habit, and probably these birds migrate now from 

 instinct. 



In general, it may be said that the migratory birds go in 

 spring to the region of their birth by a definite route. In 

 fact, the routes of some birds have been carefully mapped 

 out. The return in autumn is slower and the route is less 

 definite. 



Adaptations to environments and mode of living. — In 

 our discussions of birds many of the adaptations to then- 

 mode of living have been brought out, and many others 

 are obvious, hence we only recapitulate them here. 



Of course, they are preeminently fitted for an aerial life. 

 The fore limbs are modified into most ideal organs for flight. 

 The breastbone in those birds that fly is of such a shape that 

 it affords the most surface for the attachment of the great 

 muscles that move the wings. The bones of many birds 



