INSTINCT AND INTELLIGENCE IN BIRDS 139 
The wood swallows are social, gregarious birds of rather small size, 
characteristic of the tropical forests, where they feed upon insects, and 
often “hawk” them, like the swallows and swifts. Many have the 
curious habit of “swarming,” or clustering in cold or wet weather in 
sheltered places or under trees, possibly for’ the purpose of keeping 
warm, though this appears to be an assumption; when thus bunched, 
they crowd one upon another, all heads up, thus forming a great ball- 
like mass several feet in diameter; if disturbed they go off with start- 
ling effects produced by the whirring of many wings, often leaving, it 
is said, a few dead ones behind, which might have been smothered in the 
press. All this is suggestive of rheotropism, or the tendency shown by 
many fish, insects and other invertebrates to orient themselves in re- 
sponse to currents of air or water, and in particular of the clustering 
tendency shown by the young of many aquatic animals, as well as by 
many flying insects. Whatever its history may prove to be, no one can 
doubt that the act is purely instinctive in origin. We are reminded of 
the swarming habits of chimney swifts, which have been known to enter 
hollow trees in great numbers for the purpose of roosting and passing 
the night, especially after their arrival in spring and before their fall 
departure. 
Hornbills are large birds of peculiar structure, and wide distribu- 
tion in the old world, being noted for their great serrated bills, which 
in many of the species are surmounted by a remarkable casque or hel- 
met. But it is in the cyclical instincts of their reproductive period 
that we find the most extraordinary departure from the common type. 
Before she is ready to lay her eggs, the female hornbill enters some 
suitable cavity, in a dead tree or branch, and with or without the assist- 
ance of the male, proceeds to wall herself in, closing the opening with 
mud or excreta, or with both, with the exception of a hole large enough 
to admit the bill, and the food which is passed in by the male. While 
thus confined, the female lays the eggs, incubates them, and through 
the cooperation of her mate their naked and helpless young are reared 
until ready for flight; then the prison-house is suddenly burst open, 
the enfeebled mother and the young are liberated, and the happy 
family united in the bright world outside. Further, at intervals dur- 
ing this period the male casts off and regurgitates an inner layer of 
the gizzard, which with all the contained food comes up like a dump- 
ling, that is to say, a package or thin-walled sac, three inches long by 
two inches in diameter, and upon this generous food-supply the female 
is able to subsist for some little time. 
The practise of closing the opening to the nest is to be regarded ag 
a modification of the nest-building instinct, and while its history has 
no doubt been lost in the remote past, it may be compared with a not 
wholly dissimilar practise of the European nuthatch, which also nests 
