XIV MIGRATION 367 
Woodpigeons come to us in thousands, attracted by 
our green crops or by the large supplies of acorns to 
be found in our woods. No one who in winter sees 
the clouds of them in the sky can doubt that our native 
birds have been reinforced from abroad. Hertford- 
shire in the winter of 1893 was literally invaded by 
Woodpigeons, and the acorns almost always to be 
found in their crops showed what the attraction had 
been. 
It will be seen from what has been said that migra- 
tion is far more general than is usually supposed, and 
it is certain that even now ornithologists have not 
succeeded in observing all the smaller movements 
which change of season causes among our resident 
species. 
The Nesting-places of Migrants. 
All migrants without exception nest in the coldest 
part of their range. They pass the summer in the 
north and the winter in the south. Birds that breed 
in the tropics are resident there with the exception of 
some that nest at great elevations among mountains, 
It is quite possible that there is a migration to the 
Antarctic regions and in this case too the breeding- 
place is in the colder, though in the southern, part of 
the range! It has been thought that our birds on 
reaching Africa nested a second time. When our 
Swallows arrive at Natal, the resident birds are 
beginning to build, for their summer is beginning. 
But it is almost certainly untrue that the birds from 
1 See Hudson’s Naturalist in La Plata, 
