78 THE BIOLOGY OF BIRDS 



through the air in a line more or less parallel to its surface. 

 The gain in resistance to the wing-stroke increases with 

 the square of the velocity of the wing in a direction parallel 

 with the axis of flight." 



The steering laterally in ordinary flight is mainly due to 

 differential action of the two wings ; rapid rising or falling 

 to avoid an obstacle is mainly due to movements of the tail 

 feathers and to slight displacements of the shoulder-joint 

 forwards or backwards, the lower end of the coracoid 

 moving in a deep groove in front of the sternum. 



§ 5. Sailing Flight 



The most remarkable mode of flight is that called 

 " sailing," so well illustrated by albatross and vulture. It 

 is characterised by prolonged circling without active strokes 

 of the wings, yet without more than temporary loss of 

 velocity or altitude. The albatross describes magnificent 

 ellipses round and round the ship, apparently without 

 effort, apparently without a stroke of its wings for half an 

 hour at a time. It cannot be said that sailing flight is more 

 than partially understood. The following statements may 

 be ventured upon : — 



(i) Sailing flight is characteristic of birds that have a 

 large sail-area in proportion to total weight, such as albatross, 

 vulture, stork, and crow. 



(2) Sailing flight is not exhibited unless there is some 

 breeze, and of course there may be considerable currents 

 overhead when it is relatively still on the ground. But there 

 can be no sailing in a dead calm. 



(3) There are no visible strokes of the wings, but there 

 is a frequent change in the tilt of the body which may be 

 referred to movements of tail feathers, shoulder-joint, 

 neck, and head. There is wheeling as well as tilting when 

 the bird turns in its course and proceeds in the opposite 

 direction ; for it is certain that the sailing bird flies alter- 

 nately with the wind and against the wind. Besides move- 

 ments of parts of the body in relation to one another, there 



