276 SPOLIA ZEYLANICA. 
may have something to do with it. The torrid and often 
parching tropical summer in many localities may lower the 
food supply to someextent. Again, it must be borne in mind 
that some northern migrants winter far to the south of the 
Equator, and may be warned to turn north by the approach 
of the southern winter. In many cases, however, these 
factors do not come into force, and it is probable that the 
breeding instinct is the main impulse which calls birds back 
to their summer haunts. Most birds seem greatly attached to 
particular nesting sites which they occupy year after year. 
Professor Newton records* a remarkable instance of such a 
persistent habit : “A pair of Stone Curlew—a very migratory 
species, affecting almost exclusively the most open country— 
were in the habit of breeding for many years on the same spot, 
though its character had undergone a complete change. It 
had been part of an extensive and barren rabbit warren, and 
was become the centre of a large and flourishing plantation.” 
It is undoubtedly the fact that many birds which have a 
comparatively restricted breeding area scatter widely during 
the winter. For instance, the Curlew Stint, Tringa subarquata, 
breeds only within the Arctic Circle cn the Siberian Turdras, 
while in winter it is found all over the trepics, and occurs as far 
afield as Patagonia, Tasmania, and Cape Colony. Probably 
it is this attachment to accustomed breeding grounds that 
prevents northern migrants which visit the Southern Hemi- 
sphere from breeding a second time in their winter quarters. 
A not inconsiderable number of such species 
South Africa—which, of course, is then enjoying summer—at 
a time when the local residents are breeding. In spite of such 
favourable conditions, there is as yet no positive evidence 
that these northern migrants turn their “ winter” into a 
second breeding season. The breeding impulse does not seem 
to be awakened until the season for their migration north- 
wards sets in, by which time conditions in the South Temperate 
Zone are unfavourable. 
As regards the origin of the migratory habit, however 
instinctive it may now be, it must have originated in an 
intelligent movement intended to escape some danger or secure 
c 
‘winter ”’ in 

* « Dictionary of Birds,”’ p. 553. 
