338 



ZOOLOGY. 



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. 



Fig. 326. The Owl (Scops Vulgaris) . 



Fig. 327. Bird of Paradise. 



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 experi- 

 ments made oil the young of mi- 

 gratory 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 



* Extremely doubtful. K. K. 



1 



Fig. 328. Sitta; Nut-hatch. 



