INCUBATION. MIGRATION. 453 



of the period of incubation is equally various ; it seems to 

 depend upon the size of the egg and the degree of development 

 of the young when hatched. 



Incubation essentially consists in keeping the eggs at a warm, 

 uniform temperature ; this is effected by the body of the sitting 

 bird, and is often facilitated by the presence of naked places on 

 the body. As a rule, the mother alone sits, and the male occu- 

 pies himself with bringing her food. Not unfrequently, how- 

 ever, as in the pigeons, lapwings, and many swimming birds, the 

 two parents relieve one another regularly, and in many Limi- 

 colae the male seems to do all the sitting. In the ostrich the 

 female alone sits during the first period of incubation ; later 

 the parts are changed, and the male undertakes the chief part of 

 the incubation, especially sitting almost all night. The beha- 

 viour of the cuckoos of the Old World and certain grackles (Molo- 

 brus) of the New is very remarkable ; they leave the building 

 of nests and the care of their brood to other birds, and place 

 their small eggs, singly, amongst the eggs of various birds. 



Leaving out of consideration the activities which relate to 

 reproduction, the instinct of birds manifests itself, principally 

 in late summer and autumn, as an impulse to migrate, and still 

 more mysteriously as a true guide on the journey. Few birds 

 of the colder and temperate climates pass the winter in the 

 places where they breed (resident birds). Many of them rove 

 over larger and smaller regions in search of food. Others 

 migrate before the beginning of the cold season of the year, 

 when nourishment is deficient, from the northern climates to the 

 temperate, from these to southern regions. In the majority of 

 cases when a bird is resident as a species, it is migratory as an 

 individual. It is also to be noted that a vast number of birds 

 ordinarily strictly diurnal in their habits, chiefly if not always 

 migrate by night.* 



There are but scanty materials for the geological history of 

 this class. The oldest known bird Archaeopteryx of the Jurassic 

 formation is typically avine. From the Cretaceous, remains 

 of toothed, swimming and wading birds are known (Hesperornis, 

 Ichthyornis, etc.). In the Tertiary Period the remains are more 

 numerous and belong to groups now existing. 



More than 12,000 species of birds have been described. The 



* See Reports of the Migration Committee of the British Association. 



