296 BIRDS 



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

 central and northern Unitetl States go to Mexico and Cen- 

 tral America in the autumn, to remain dvn-ing (he winter. 

 returning again in the sjiriug. Tlie extent of the migra- 

 tions of different jjirds 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 liirds, remain throughoid the year in the same 

 localities in which they are bretl and reared. 



It is probalde that the original cause of these migrations 

 was a change in climate whereby there came aljout a 

 scarcity of food. The birds were thus forced to journey 

 to other regions in search of something to sustain life. 

 The wandering, thus induced 1)}' f(_iice, has now Ijeen regu- 

 larly practiced b)^ so many generations that it has become 

 a fixed habit, and probably these jjii'ds migrate now from 

 instinct. 



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

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

 fact, the routes of some birds have been carefully mapped 

 out. The return in autunur is slower antl the route is less 

 definite. 



Adaptations to environments and mode of living. — In 

 our discussidns of liirds man}' of the adaptations to their 

 mo(l(> of living have been brought out, and many others 

 are oln'ious, hence we (inly recapitulate them hei'c. 



Of cour.sc, they arc preeminentl>- fitted fnv an at'rial life. 

 The fore limlis are modified into most ideal organs fdi' flight. 

 The l)reastl>one in those birds that fi>' is of such a. shajie that 

 it affords tlie most surf;ice for the att.n.chment (if (he great 

 muscles that move the wings. TIr' bones of many birds 



