366 THE STRUCTURE AND LIFE OF BIRDS cuHapP. 
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 eastas 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 alsocome 
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, 
