THE SONG OF BIRDS. 625 



sometimes it is more oblique, and then the bird speeds onwards ; usually 

 both directions are combined. The raising of the wing after each stroke 

 requires relatively little effort, the resistance to be overcome being very 

 slight. In steering, the feathers of the tail often bear to the wings a 

 relation comparable to that between rudder and sail. 



Modes of flight. — There are three chief modes of flight : — 



(1) By gliding or skimming, during which the bird has its wings 

 spread, but does not flap them, depending for its movement on the 

 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. It has been carefully studied 

 and photographed by Marey in the gull and pigeon. 



(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, 

 and albatros, and has been observed only when there was wind. It is 

 still imperfectly understood, but there seems most to be said in favour of 

 the theory that it depends on the varying velocity of the wind at different 

 heights. 



The Song of Birds. — Singing is a natural expression of 

 emotional intensity. The song rises in the bird, Richard 

 Jefferies said, as naturally as the sap in the bough. It is 

 richest at the climax of emotion in 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 

 alone. The twittering of swallows, the cawing of rooks, the 

 melancholy voice of the sea-mew, the lapwmg's prayerful 

 cry, the weird call of the curlew, are familiar to most of us. 

 A few birds, notably the parrot and the jackdaw, can be 

 taught to pronounce articulate words : but the power of 

 imitation is widespread among birds, the case of the canary 

 learning the song of the nightingale being a well known 

 instance. This power of imitation has some importance in 

 relation to the general theory of instinct, for the song of all 

 birds is probably in great part imitative, though to a certain 

 extent the musical talent is really inherited. Young birds 

 taken away from their nests when very young, so that they 

 have hardly heard the voices of their kind, will sing the 

 characteristic song of the species, but do so imperfectly. 



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