MIGRATION OF BIRDS. 295 
You may find little groups of such birds in suitable places 
on the Ceylon lagoons during the south-west monsoon, and it 
is important to note that the great majority of such birds have 
not changed into breeding plumage. This fact strengthens 
the evidence for the theory that it is the breeding impulse 
which calls migrants north in the spring. Birds which for 
some reason or other have not developed full maturity do 
not feel the impulse to such a degree, and so some of them 
remain with us. 
This habit of loitering has been known for some time, but 
no one seems to have studied it with great thoroughness, pro- 
bably because most of the work on migration has been carried 
out in temperate climates, where loiterers do not attract so 
much attention. I think it is a subject which will well repay 
investigation, and here in Ceylon we have an ideal opportunity 
for its study. 
It is at the southern limits of the migration routes that the 
question of actual loitering of migrants in their winter quarters 
can best be investigated. Then there is no disturbing factor 
introduced by birds of passage. Where such a southern 
limit lies far south of the Equator, as in South Africa, much 
loitering is not likely to take place. The approach of the 
southern winter would drive north those birds which might 
otherwise be inclined to stay. In our case we are at the very 
end of a migration route, and in the tropics, with an equable 
climate, a combination of conditions which affords the fullest 
temptation for birds to linger. 
It appears to me that in loitering we may see the inter- 
mediate steps by which all along the tropics new resident 
species are being evolved from northern forms by the gradual 
breaking down of the migratory habit among a proportion 
of the birds of any species. When birds once take to loitering, 
they find that life in the tropics is possible all the year round, 
and so in process of time may slowly be induced to overcome 
the deeply ingrained instinct which impels them to migrate 
northward for the purpose of breeding. In some cases, from 
whatever reason, this evolution has already taken place, and 
accounts for the interesting group of our “ partial migrants,” 
to which I have already alluded. 
