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 startling 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 response 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 distribution in the old world, being noted for their great serrated bills, which in many of the species are surmounted by a remarkable casque or helmet. 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 assistance 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 during this period the male casts off and regurgitates an inner layer of the gizzard, which with all the contained food comes up like a dumpling, 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 as 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
