392 MIGRATIONS. SOCIABILITY. GENERAL HABITS. 



before. Spallanzani, a celebrated Italian physiologist of the last 

 century, saw the same couples return to their old nests, almost 

 without taking the trouble to repair them, for eighteen years 

 consecutively. Swallows show, on other occasions also, a singular 

 power of directing themselves towards a particular place, from 

 which they are at a considerable distance. If a hen that is sitting 

 is carried to a distance, shut up in a cage, and her liberty is then 

 given her, she first rises to a considerable height, as if to recon- 

 noitre the country, and then flies in a straight line to the spot 

 where she has left her brood. Spallanzani repeated this experi- 

 ment successfully on several occasions ; and saw a couple of 

 River-Swallows that he had carried to Milan, return in thirteen 

 minutes to their young ones left at Pavia. 



357. The instinct of sociability is also much developed in 

 certain Birds ; we have elsewhere had occasion to speak of the 

 manner in which several of these animals unite in the construction 

 of their habitations (ANIM. PHYSIOL., 710) ; and we shall bring 

 together, in the later portion of this work, some facts yet more 

 remarkable, in regard to the association of Birds for particular 

 purposes ; but we must at present be content with observing, 

 that this instinct is scarcely ever developed, except in species 

 living on insects or vegetable substances ; and that Birds of Prey 

 live almost always solitary, or united only in pairs. 



358. Birds, as well as Mammalia, vary in the manner of 

 procuring their food ; most seek it only by day ; but some are 

 nocturnal or crepuscular (flying only by twilight) ; and it is 

 worthy of remark, that they have generally dark colours and 

 downy plumage, so that their wings strike the air without 

 noise ; as if the Creator, in his infinite providence, had wished 

 to favour these Birds in their nocturnal pursuit of food. The 

 several species of the families of Owls, Goatsuckers, &c., present 

 examples of this coincidence, between the habits of the Bird, 

 and the nature of the plumage. 



359. The number of species of Birds known to naturalists is 

 about five thousand ; and, as the organisation of these animals 

 presents great uniformity, the classification of them offers con- 

 siderable difficulties. The characters employed to divide them 

 into orders, families, and genera, are furnished principally by 



