THE VERTEBRATES 435 



in the previous year had experience in migration. Probably 

 in most cases the young birds hatched in northern regions 

 learn the way southward by accompanying older birds. 



Migration southward is believed to be stimulated by ab- 

 sence of food-supply in winter ; and a desire to re-occupy their 

 old haunts and breeding places leads to return in spring. 



Some birds may be seen at given localities in any month 

 of the year, and such species are said to be resident. Many 

 birds which are resident as species migrate as individuals; 

 and those seen in winter have often come from the north, 

 while the summer residents of the same species have moved 

 southward. 



The migration is not always due north and south. Coast 

 lines, mountains, and great rivers may cause eastward or 

 westward deviations. These are not yet well understood. 



The instincts connected with nesting habits of birds have 

 long excited the wonder of ornithologists. Many birds build 

 nests exquisite in form. Each species has its peculiarities 

 as to choice of nesting site. Why quails nest on the ground, 

 swifts in a chimney, swallows beneath the house-eaves, and 

 orioles in hanging nests cannot be satisfactorily explained; 

 and we must be content with saying that these birds have 

 inherited their nest-building instincts from their ancestors. 

 How and why their ancestors learned to build nests in certain 

 ways and in specially selected positions is entirely a mystery. 



The instincts connected with incubation of the eggs and 

 caring for the helpless young are no less remarkable. In 

 some species the female broods the eggs and her mate brings 

 food to her. In others the male and female take turns in 

 brooding the eggs. Certain cuckoos and some other birds 

 have the peculiar habit of avoiding the trouble of brooding 

 by placing their eggs in the nests of other birds, which 

 incubate them. In many species both male and female 

 cooperate in the work of collecting food for the young 

 nestlings. 



