70 LIFE : OUTLINES OF GENERAL BIOLOGY 



Let us now focus the familiar fact that the birds of a north 

 temperate country may be classified, as regards their migration, 

 into five groups. First there are the summer visitors, such as 

 swallow and swift, cuckoo and nightingale, who arrive from the 

 south in spring, nest within our bounds, and return in late summer 

 or in autumn to their southern and south-eastern winter quarters. 

 Second, there are the winter visitors, such as fieldfare and redwing, 

 snow-bunting and great northern diver, who nest in the Far North 

 but come south in winter. A few of these winter visitors may occa- 

 sionally nest in this country, as the snow-bunting well illustrates. 

 Third, there are the birds of passage in the stricter sense, like some 

 of the sandpipers, the great snipe, and the little stint, which usually 

 rest for a short time only in a country like Britain, on their way 

 farther south or farther north, as the case may be. Fourth, there are 

 the "partial migrants", a group larger than used to be supposed. 

 Partial migrants are those birds which are never unrepresented in 

 the country in question, yet some go while others stay. Thus in 

 many parts of Scotland there is not a month in the year when 

 lapwings are not to be seen, yet the "ringing method" has proved 

 that there is a regular autumnal migration from the sterner Scotland 

 to the more genial Ireland. Similarly, there arc always goldfinches 

 in the south of England, yet there is a regular migration southwards 

 in October and a corresponding return in April. Fifth, there are the 

 strictly resident' birds, such as, in Britain, the red-grouse and the 

 house-sparrow, to take a sacred and a profane example. It is not 

 meant that even the red-grouse remains stationary, year in and 

 year out; but, apart from artificial intr» Auctions, it is not known 

 outside of Britain. On the other hand, although the hedge-sparrow 

 may be called resident in Britain, it is migratory on the Continent. 

 Many of our so-called resident birds turn out to be partial migrants, 

 as is true of rooks, skylarks, and song-thrushes. It is well to avoid 

 thinking of these convenient groups in any hard and fast way, for 

 a species that is resident in one part of the country may be a migrant 

 in another; and the summer visitors of a northern country are the 

 winter visitors of a southern one; or vice versa. To the five groups 

 referred to, namely, summer visitors, winter visitors, birds of 

 passage, partial migrants, and residents, there may be added a group 

 for "casual vagnmts" such as the North American killdeer-plover, 

 which has been recorded a few times in Britain, or better-known 

 stragglers, such as Pallas's sand-grouse and the waxwing. 



Since there are hundreds of different kinds of birds in the 

 Northern Hemisphere, and a great variety of climatic and other 

 conditions, it is well to avoid rigidity in our pictures of bird- 

 migration. Thus, while there is a clear contrast between the spring 

 movement northwards /rom warm countries and the more impressive 

 autumn movement southwards to warm countries, it must be 



