THE ENGLISH SPARROW AND ITS ALLIES 309 



Bird Migration. The birds of any territory may be 

 classified on the basis of their residence into four groups : 

 permanent residents, winter residents, summer residents, 

 and transient visitants. In NCAV York the permanent 

 residents are represented by the screech-owl, the crow, 

 and the gold-finch. Winter visitants are represented by 

 the snow-bird and white-throated sparrow ; summer resi- 

 dents by the wood-thrush, cat-bird, yellow warbler, bobo- 

 link, and barn-swallow ; transient visitants by most of the 

 warblers, the fox-sparrow, and white-bellied swallow. As 

 this classification indicates, the bird fauna of any place is, 

 to a great extent, shifting from season to season. In the 

 spring at any latitude many species are seen passing far 

 north, and in the autumn passing south again. Tem- 

 perate as well as southern latitudes receive in the winter 

 time the southern edge of a more northern fauna. Thus 

 geese and ducks are forced by the universal ice of the 

 Arctic winter to go to regions of open water, at least. 

 Many kinds of birds which spend the winter in the South 

 come North to breed. For example, the bobolink is found 

 nesting in the New England fields in summer, and is then 

 altogether absent in the South. In the fall this bird is not 

 found in New England, but occurs in the Gulf States as 

 the rice-bird. The cause of this migration on the part of 

 birds breeding in the North is the need of food. Insectiv- 

 orous birds, especially, could not obtain food enough in 

 the North in winter. 



Migration Routes. It is known that many species 

 which migrate even great distances travel along well- 

 defined paths. In Europe the paths of the shore migrants 

 have been carefully mapped out. It appears that these 

 birds follow the shore line very exactly, except in some 



