The Tetrapoda: Amphibia, Reptiles, Birds 621 



other and engage in feathery duels. Some of the tropical birds such as 

 the birds of paradise of New Guinea have even more elaborate rituals. 



In nest building and care of the young, great differences occur 

 among the various bird species. A few birds such as the cowbird of this 

 country and the cuckoo of Europe disdain all care of their young. They 

 lay their eggs in the nests of other birds, and their precocious young 

 literally push the hosts' offspring out of the nest. The mourning dove 

 makes a careless nest of hastily thrown together twigs, a striking con- 

 trast to the elaborately woven, hanging nest of the oriole. The birds of 

 the prairies construct their nests of grasses and feathers directly on the 

 ground. In Africa, one bird, the weaver bird, builds communal nests 

 in which many pairs carry on their nesting activities at the same time. 



Both in the number of eggs and the care and development of the 

 young differences likewise occur. The young of most ground-nesting 

 forms hatch in a very mature condition, and can run about searching 

 for food and shelter as soon as they emerge from the eggs. These are 

 called precocial birds. They contrast with the condition found in most 

 birds which nest above the ground. Among these latter, the young 

 hatch in a very immature stage and require considerable parental care 

 before they are able to fly. A few such as the group to which the 

 doves belong secrete a special substance known as pigeon milk for the 

 nourishment of their young. Such helpless young are known as altri- 

 cial birds. 



In many species, the incubation and care of the eggs is a task for 

 both parents ; in others only one takes charge. Some species map out 

 territories which they defend vigorously against intruders and es- 

 pecially against birds of their own species. The house wren is a par- 

 ticularly noisy territorial defender. 



Migi'ation.— One of the best known and least understood aspects 

 of bird behavior is that of migration. Migration is very limited in some 

 species ; for example, the bird may simply move up and down a moun- 

 tain in an altitudinal movement corresponding to the changing seasons. 

 Others may move only a few hundred miles from one area to another. 

 Usually these movements are in a north-south direction in the fall and 

 the reversal in the spring. 



The majority of the smaller birds of the United States and Canada 

 spend their winters in southern Mexico, Central America, or northern 

 South America. In performing these long migrations, they usually 

 follow certain definite routes known as jlyzvays. . In the United States 



