274 SPOLIA ZEYLANICA. 
As regards migration, the birds of a temperate climate, such 
as the British Islands, may be divided into five main classes :— 
(1) Permanent residents, for instance, the Grouse, though 
even the Grouse has a short local migration from the moors, 
where it breeds, to the valleys, where it spends the winter. 
(2) Summer residents, such as the Cuckoo, which spend the 
winter in warmer countries. 
(3) Winter residents, such as the Fieldfare and Swan noted 
above. These birds breed in the far north and come in winter 
to the British Isles as a haven of comparative warmth. 
(4) Birds of passage. In the spring hosts of birds which 
neither breed nor winter in the British Isles halt there for 
short intervals en route from their winter quarters in the 
south to their breeding stations in Scandinavia, Iceland, and 
Greenland. In the autumn the process is reversed. 
(5) Irregular migrants and stragglers. These are birds 
which occur only at irregular intervals. Some species turn up 
fairly often in more or less regular cycles ; others, again, are 
vagrant oceanic rovers, such as some of the rarer Petrels, 
while yet others are storm-driven wanderers. 
It must not be imagined that each species has a hard and 
fast place in only one of the above classes. Numbers of the 
commonest English species, such as the Song Thrush and 
Skylark, which many people look upon as permanent fixtures 
in the English landscape, have the most complex movements. 
Some Song Thrushes are perennial residents in the British 
Isles, though even among these there is a vast amount of local 
movement from the more exposed localities, where they breed, 
to more sheltered valleys, where they retreat in cold weather. 
Many of the Thrushes, however, are only summer residents, 
and winter in countries south of the Channel. Their place in 
winter is taken by individuals which have bred in Scandinavia 
and Central Europe. Other Thrushes, again, winter in France 
and Southern Europe, breed in Scandinavia, and occur on 
English shores as birds of passage. Lastly, birds which 
usually winter in Central Europe may occasionally be driven 
out of their accustomed winter haunts by unusually severe 
weather, and may seek temporary refuge from the cold snap 
in the milder English climate. Indeed, as Mr. Eagle Clarke 
