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 jyurpose 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 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 



