FLIGHT. 



617 



Fig. 272. — Wings coming down. — From Marey. 



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 compar- 

 able to that be- . ^iV&tfA^" 

 tween 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 ac- 

 quired by previous 

 strokes, by de- 

 scending 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, back- 

 ward 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, and albat- 

 ross, and has been ob- 

 served only when there 

 was wind. It is still 

 imperfectly understood, 

 but it probably depends on the varying velocity of the wind at different 

 heights. The bird probably sails along the line of two currents of 

 different velocity. 



Song 1 of birds. — Singing is a natural expression of emotional 



Fig. 273. — Wings completely depressed. 

 — From Marey. 



