40 
Introduction: Migration. 
of these white birds, of their fairly swarming at times on other islets, of great 
flocks literally hanging in clusters on the trees of the little island of Manado 
tua. So, too, the Nicobar Pigeon, Caloenas^ is described as swarming by 
thousands upon its almost inaccessible breeding islet of Batty Halve in the 
Nicobars. It is obvious at once that these small islets cannot and do not 
support such a population of Pigeons for long, consequently the birds are 
repeatedly on the move, flying over the sea in search of fresh feeding grounds. 
Instances of their being seen in the act of crossing the sea are given in the 
text (p. 629, 659). It appears that they visit their breeding islands seasonally, 
but everything has still to be learnt about the periodicity, if any, of their visits 
to the other islands of their ranges. The four Pigeons in question have no 
near affinity with one another ; the Nicobar Pigeon is the sole representative of 
a subfamily; the other three have more or less near affinities with mainland species. 
As is well known, certain sea-birds, such as the Gannets, Albatroses, and 
some Terns, resort to particular rocks or other islets to breed. In their case, 
as in that of the Pigeons, protection of their brood from animals destructive to 
their eggs and young may well have been the original motive for the adoption 
of these habits, yet with the Pigeons it remains strange that they avoid the main- 
land after their young have been safely reared. (For further remarks hereon, 
see pp. 616, 629, 659 — 661). 
For the sake of the general reader, who may be apt to suppose that narrow 
straits of the sea offer no barrier to the geographical distribution of tropical 
species, it may be mentioned that, so far from this being the case, there is 
reason to believe that resident species never, or only very exceptionally, cross 
the sea; were it otherwise the species would not be found with such restricted 
ranges as is actually the case. 
Migration proper in the East Indian Archipelago. — The following is a list 
of most of the more prominent migratory birds of Celebes. A few species, 
well known to be migrants, offered difficulties which have led to their being 
omitted, while a large number of other species have been left out because their 
migrations are as yet hidden in such obscurity that it would probably be mis- 
leading to attempt to trace them categorically. In the case of the species given 
it has often been impossible, in the absence of positive data, to avoid speculation 
in the use of the signs for summer and winter haunts, but it will generally 
be found that too little has been said, rather than too much. 
O: signifies “summer visitor”. (It would generally be safe to assume 
that the species breeds in the localities so marked, but in not one- 
half of them have the nest and eggs yet been discovered.) 
X: signifies “winter visitor”, or “passes through on migration”. 
+ : signifies “rare”, or “a straggler”. 
