ii8 THE MIGRATIONS OF BIRDS 



is confined within a limited area for the period of 

 winter, and then spreads to reoccupy the full range 

 with return of warmer weather. 



The difference between subspecies has made pos- 

 sible the discovery of interesting facts indicative of 

 length of migration in a number of species. As an 

 example may be cited the yellow-throat of Florida 

 {Geothlypis trichas ignotd)^ marked from other forms 

 by a more rounded wing, which is strictly sedentary, 

 but in winter has as its companions yellow-throats 

 from the north, some of which cross to the Bahamas 

 and Cuba. In England, an insular area where such 

 matters are readily detected, resident forms of the 

 greater spotted woodpecker, the dipper, the robin 

 red-breast, the song-thrush, the great titmouse, and 

 others, are joined in winter by individuals of con- 

 tinental subspecies from the north, which return to 

 their proper haunts at the opening of the breeding 

 season. Similar instances may be found in many 

 areas in the temperate zones both north and south 

 of the Equator. 



Very often individuals of a widely distributed 

 species from the northern part of the range tend to 

 migrate farther south than those from intermediate 

 regions, instead of there being a uniform shift of 

 northern birds to the central portion of the range, to 

 replace others that have gone on southward. In 

 other words there is a tendency for birds living 



