8 THE MIGRATION OF BIRDS 



snipe, which do not nest in Britain, must be in- 

 cluded many (for example the robin, rook, song 

 thrush and common snipe) which are also permanent 

 residents. 



4. Birds of Passage or Spring and Autumn 

 Migrants : birds which neither nest with us nor 

 normally remain for the winter, but merely use the 

 British Islands as feeding and resting places on 

 their journey between the northern breeding area 

 and the southern or eastern winter quarters. This 

 group is an especially difficult one, for in it must 

 be included such birds as dunlins and curlews, 

 which are represented as breeding species in Britain, 

 and also a number of birds which apparently go 

 no further south than our islands in winter, and 

 others which, though not breeding, go no further 

 north in summer. The actual status of these 

 individual birds is uncertain. In this group too 

 we have the Greenland wheatear, so closely allied 

 to our familiar early migrant that, unless the bird 

 can be measured, its identification is uncertain. 



5. Irregular Migrants : birds which may be classed 

 in other groups. Some of these are really winter 

 residents, but their visits are so irregular that they 

 may for convenience be classed with spasmodic or 

 occasional invaders, such as Pallas 's sand-grouse, 

 which arrive at uncertain intervals in large numbers. 

 Some of their number, during these irruptions, usually 



