758 



PHYLUM CHORDATA I CLASS AVES— BIRDS 



velocity acquired by previous strokes, by descending from a higher 

 to a lower level, or by the wind. This may be readily observed in 

 gull and heron, in a pigeon gliding from its loft to the ground, or in 

 a falcon swooping upon its quarry. 



2. By active strokes of the wings, in which the wings move down- 

 ward and forward, backward and upward, in a complex curve. This 

 is of course the commonest mode of flight. 



3. By sailing or soaring with motionless spread wings, in which the 

 bird does not necessarily lose in velocity, or in vertical position, as is 

 the case in gliding. It is illustrated by such birds as crow, falcon, 

 stork, albatross, and has been observed onlv when there was wind. 



Fig. 455. — Position of wings in pigeon at maximum elevation. 



— From Marey. 



Song of birds. — Singing is a natural expression of emotional 

 intensity. It is richest at the breeding season, and is always best and 

 often solely developed in the males. But song in any excellence is the 

 gift of comparatively few birds, though nearly all have a voice of some 

 sort, often so characteristic that the species may be recognised by its 

 call. The parrot and the jackdaw, and others, can be taught to 

 pronounce articulate words ; and the power of imitation is widespread 

 among birds, which are notorious plagiarists. This power of imitation 

 is important in relation to the general theory of instinct, for the song of 

 all birds is prt)ba])ly in great part imitative, though to a limited extent 

 inherited. Young birds taken away from their nests when very young, 



