FLIGHT OF ANIMALS — GRAY 297 



yet all the conditions of balance must also be under the command of 

 the pilot, for by control of these he must fly, and fly as he decides to. 



Similar problems must be solved by birds. A bird achieves con- 

 trol partly by using the tail as a horizontal rudder, and partly by 

 changing the shape and posture of the wings; and these movements 

 are also under the immediate control of the semicircular canals of 

 the bird's ears, just as are the movements of the fins of a fish. 



A bird flying — propelling itself by its own muscular efforts — is one 

 of Nature's great masterpieces, and she guards very closely the 

 secrets of her success. It is easy to see that the bird's wings are 

 beating upward and downward and to realize that these movements 

 must provide a lifting force equal to the weight of the body, and a for- 

 ward thrust equal to the backward drag of the air. From a mechanical 

 point of view, the wings of the flying bird are carrying out simultane- 

 ously the functions performed respectively by the wings and the pro- 

 peller blades of an airplane ; a bird's wings are, therefore, more closely 

 comparable with the rotor and screw blades of an autogyro or heli- 

 copter. 



But any attempt to make an accurate study of the movements of 

 a flying bird meets with difficulties. The form of the wing, for in- 

 stance, is constantly altering during the course of its beat, some of the 

 changes being due to the suppleness of the wing feathers and others 

 to the bird's own internal muscular movement. Then again, the bird's 

 speed and the frequency of its wing beats are too great for the wing to 

 be exactly observed by the human eye. As a general rule, the smaller 

 the bird the faster and more often it beats its wing; a swan or a heron 

 beats its wings about 2 times, a gull 5 times, a pigeon 10 times, and a 

 hummingbird 50 times per second. Details of such wing movements 

 can only be found out by high-speed cinematography using a special 

 light that gives a succession of very bright and very quick flashes. The 

 photographs shown on plates 5-8 were taken in this way ; they show 

 the movement of the wings of a pigeon flying slowly across a room. 



In plates 5 and 6 a pigeon is flying toward the camera. The down- 

 stroke begins as shown by photograph 1 with the wings fully ex- 

 tended, stretched up over the back of the bird. Both wings then strike 

 downward (photographs 2-4) with the whole surface of the wing 

 moving down almost at right angles to its horizontal path of mo- 

 tion; the primary feathers are bending upward under the pres- 

 sure of the air (photograph 3) . Wlien the wings have reached a hori- 

 zontal position (photograph 4), the downward motions cease and 

 the wings swing forward, to meet in front of the body (photo- 

 graphs 5 and 6) ; during this change of movement tlie primary feath- 

 ers separate from each other, and bend sharply upward under the 

 lifting effect of the air moving between them. Photograph 7 shows 



