366 THE STRUCTURE AND LIFE OF BIRDS chap. 



drive away the young ones, in order, it is believed, to 

 avert the evils of over-population. If hard weather 

 comes, the old birds also move southwards, till only 

 those that are fed by their human friends remain. 

 On the Continent almost all migrate, travelling as far 

 south as the Sahara or as far east as Turkestan. Thus 

 species which are resident in one country may be 

 migratory in another. In this respect England with 

 its mild winters is most fortunate, for the proportion 

 of residents is with us much greater than it is in 

 other countries where the cold is sharper. According 

 to Dr. Wallace, in Massachusetts less than one-third 

 of the birds are resident, in England more than two- 

 thirds. Among our partial migrants is the Thrush. 

 Every autumn our native Thrushes are joined by 

 large numbers from the north, which soon however 

 pass on further south and take many of the British- 

 born birds with them, leaving the species almost un- 

 represented in some parts of the country. In Germany 

 not one is left, but all go south. Our Blackbirds too 

 are perhaps partial migrants. Large flocks visit us 

 in autumn and it is probable that some of our native 

 birds leave our islands in winter. Starlings also come 

 to us in large numbers as autumn visitors and, as a 

 rule, continue westwards, leaving some parts of the 

 country untenanted, while the south of Ireland is 

 thickly peopled with them. The Wild Duck, the 

 Common Snipe, the Woodcock represent species 

 that are mainly migratory with us, only a few re- 

 maining to nest in Britain. Some birds which we do 

 not think of as travellers, such as the Moorhen, move 

 from the colder northern districts towards the south. 



