BIRD MIGRATION. .'$7 



Stairs east of the Great Plains; but an arm of the breeding range 

 extends northwestward to the Pacific coast in British Columbia (see 

 fig. 18). It is evident that this is a late extension of the range, that 

 it has taken place by a westward movement from the lower Missouri 

 Valley section, and that the nesting birds of Washington and British 

 Columbia retrace in spring and fall the general route by which they 

 originally invaded the country. 



The origin of this vireo's route is also indicated by the isochronnl 

 lines shown in figure 19. On March 20 the vanguard is just entering 

 the United States from the winter home in South America. North- 

 ward progression is fairly uniform for the next 5 of the 10-day peri- 

 ods, carrying the birds to eastern Nebraska, southern Michigan, and 

 southern New England. But then a change becomes evident. The 

 eastern birds continue their lines of flight and pass almost directly 

 to their summer homes. Some of the western-born individuals, how- 

 ever, begin to turn at a wide angle from their previous course and 

 proceed on a long northwestward slant to the Pacific. It is especially 

 to be noted that as these individuals change their course they quicken 

 their speed until they travel on the average more than twice as far 

 a day as their eastern brethren. 



In the case of the bobolink the evolution of a new extension of the 

 migration route is now occurring before our very eyes. By nature a 

 lover of damp meadows, the bobolink was formerly cut off from the 

 Western States by the intervening arid region. But with the advent 

 of irrigation and the bringing of large areas under cultivation, little 

 colonies of nesting bobolinks are beginning to appear here and there 

 almost to the Pacific. Some of these colonies are shown by encircled 

 areas on the map in figure 1, and the probability is that the not dis- 

 tant future will witness a large increase in the number of bobolinks 

 west of the Rocky Mountains. 



NORMAL AND ABNORMAL MIGRATION. 



The relative position of the northern and southern groups of indi- 

 viduals of a species during the two yearly migrations is one of the 

 obscure points that late investigations help to elucidate. The sup- 

 position is that in the case of species which adopt what may be called 

 normal fall migration, birds which nest farthest south migrate first 

 and proceed to the southern end of the winter range ; those that breed 

 in the middle districts migrate next and occupy the middle of the 

 winter range ; and, finally, those of the northern part of the breeding 

 range migrate last and remain farthest north for the winter. In other 

 words, the migration is a southward movement of the whole species 

 during which the different groups of individuals or colonies retain in 

 general their relative positions. This has been commonly believed, 

 but only of late has it been clearly proved for any particular species. 



