POSITIONS ASSUMED BY BIRDS IN FLIGHT — BEETHAM. 437 



prevent the body from striking the water, but also help to increase 

 the velocity necessary to enable the bird to rise again. In plate 4, 

 figure 2, although the feet of the kittiwake have ceased to touch the 

 surface, the bird is still running, as it were, in space. 



Another method often practiced by birds to lessen speed is that of 

 depressing the tail, and so offering a resistance to the air rushing 

 along the under surface of the body, and this is illustrated in the 

 gannet shown in plate 5, figure 1 . This use of the tail is very similar 

 in its purpose and result to the use of the feet as brakes. Steering is 

 also, of course, aided by the tail, it being visibly turned from side to 

 side, raised or depressed, when flight is being executed amid tumul- 

 tuous currents. But this method of steering by the tail is rather 

 corrective than initiative in its use, being principally employed to 

 compensate for irregularities in the air currents. When a bird is 

 suddenly and deliberately changing the direction of its course — 

 turning an aerial corner, so to speak — the plane of the wings is 

 changed from the horizontal position assumed when gliding to a 

 more or less vertical position, the inclination depending on the 

 abruptness of the turn and the pace at which it is executed. If the 

 turn is to the right, then the left wing is raised and the right depressed, 

 and, of course, vice versa for a turn to the left. When writing here 

 of one wing being raised and the other depressed, I refer to their 

 positions relative to each other, and not to their relation with the 

 body. That is to say, the wings and body may be held rigidly in one 

 plane, the inclination of this as a whole being changed from the 

 horizontal to toward the vertical. This vertical position has been 

 almost reached by the bird, of which, unfortunately, only a portion 

 is shown, in the upper part of plate 5, figure 2. It will be noticed 

 that the left wing is depressed and the right raised ; the bird is there- 

 fore sweeping around to the left. I have seen birds when thus sud- 

 denly altering the direction of their course actually exceed the 

 vertical position, turning the plane of their surface through an angle 

 of about 105°, thus making an angle of about 75° with the horizon, 

 their backs then, of course, being on the underside. 



The question of air currents is of paramount importance in flight, 

 though it is probable that owing to their invisibility we have as yet 

 little idea of how extensive and acute these movements are. If, 

 however, we watch small companies of gulls flying leisurely in the same 

 direction, we shall often see them pass through such local air currents, 

 whose existence is plainly indicated by the sudden and harmonious 

 wheeling of the birds. It is often very noticeable, too, how precisely 

 in the same manner all the birds will compensate for the current. 

 This is suggested in plate 6, figure 1 , where the four central birds are 

 passing through a disturbance, and it will be noticed how each is 

 "trimming" for it in much the same way, even to the awkward bend 

 in the neck. 



