THE CLASS BIRDS. 305 



still greater. In the smaller species, the number of eggs 

 exceeds that of the larger; the eagle lays but one or two; the 

 tomtit and the raven, from fifteen to twenty. 



The constancy with which the parents sometimes the fe- 

 male only, sometimes also the male hatch their eggs ; the 

 care they take of their young when they appear ; the courage 

 and intelligence shown by the parent bird in defending the 

 young, are facts within the observation of all. Nevertheless, 

 there are some birds, as the cuckoo, which deposit their egg 

 or eggs in the nests of other birds ; the young cuckoo gra- 

 dually dislodges his companions from the nest, and occupies 

 the whole. It afterwards rejoins its parents, which seem to 

 remain in the neighbourhood for that purpose. Instinct leads 

 the cuckoo to deposit its eggs in the nest of some insectivorous 

 bird (that being the food on which its own young requires to be 

 fed), such as the nests of the linnet, yellow-hammer, black- 

 bird, <fec. 



Notwithstanding the strength of the instinct which leads 

 birds to hatch their eggs, it seems certain that it may be. 

 modified by certain circumstances. Ostriches, for example, 

 sit carefully on their eggs in temperate climates, but leave 

 them in tropical regions to be incubated by the sun. It 

 would seem, also, that several of these large birds unite to 

 lay their eggs in one nest, taking charge of them alternately. 



443. We have already ( 325) described at length the 

 migratory instinct of birds, which induces them to migrate 

 from one region to another at fixed seasons. The cause of 

 this is explicable in some instances, but not in others ; for 

 certain species, it may be affirmed that the causes of these 

 migrations are wholly unknown. In some experiments made 

 on the young of migratory birds, it appears that the anxiety 

 to leave comes on when these are detained in captivity and 

 plentifully supplied with food. 



Upon the whole, atmospheric changes would seem most to 

 influence migratory birds, and that such changes also modify 

 the time of arrival and departure. In each species, however, 

 the period is fixed, and may be calculated on. Occasionally, 

 age modifies the time of flight or departure, the old leaving 

 before the young. 



444. It is also a remarkable circumstance in the history 



of birds, that they can leave their nests for so great distances, 



and yet return without the smallest difficulty or chance of 



mistake. Swallows, for example, year after year (for eighteen 



x 



