254 AVES. 



nests of those Birds which glue particles of extraneous matter together 

 with their sticky saliva or which weave fine tressworks of moss, 

 wool and grass-stalks (Weavers). As a rule it is the female alone 

 which builds the nest, the male merely helping in collecting the 

 materials. There are, however, instances in which the male takes a 

 share in the construction (Swallows, Weavers) ; while in other cases 

 (Gallinacei, Chaffinch) the male takes no share at all in building 

 the nest. Many sea Birds, as the Auks and Penguins, lay but 

 one egg, the large Birds of prey, Pigeons, Swifts, and Humming- 

 birds, lay two eggs. The number of eggs is larger in the singing 

 Birds and still greater in the swimming Birds of ponds and rivers, 

 and in the Fowls and Ostriches. The duration of the period of 

 incubation is equally various ; it depends upon the size of the egg 

 and the degree of development of the young when hatched. 



While the Humming-birds and golden-crested Wrens incubate for 

 eleven to twelve days, and the singing Birds fifteen to eighteen days, 

 Fowls require three weeks, Swans double that time, and Ostriches 

 seven to eight weeks. 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 occupies 

 himself with bringing her food. Not unfrequently, howevei , as in the 

 Pigeons, Peewits, and many swimming birds, the two parents relieve 

 one another regularly ; the male in such cases certainly sits only for a 

 shorter time during the day, while the female sits through the whole 

 night. In the Ostrich the female only 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 behaviour of numerous Cuckoos, and especially of our native 

 species, is very remarkable ; they leave the building of nests and the 

 care of their brood to other Birds, and lay their small eggs, singly 

 and at intervals of about eight days, amongst the eggs of different 

 singing Birds. 



The care and nurture of the young usually falls entirely or mostly 

 on the hen bird. On the other hand, as a rule, both parents take 

 an equal part in the protection of the brood. 



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 



