358 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1939 



it, like a section of the horizon, may appear to be straight but is in 

 reality curved. If so, he knew that the wind he was riding would 

 carry him to the Mississippi flyway, that it would shift to southeast 

 before he reached the valley, then to south-southeast and south. 



This is, of course, pure guesswork and may sound visionary, except 

 that it is no more preposterous to grant the goose full knowledge of 

 horizontal winds than to give definite knowledge of vertical winds 

 to soaring birds. Obviously, a turkey buzzard gains altitude on a 

 rising current of air. Using gravity for motive power, he noses 

 down slightly in a remarkably flat glide and descends more slowly 

 than the air is rising. When he has exhausted the lift of the ascend- 

 ing current or wishes to change position, the buzzard sharpens his 

 descent and glides swiftly across the neighboring down-draft. 

 Whether he gains or loses altitude in the long run will depend on the 

 length of time he is in the opposing currents rather than on the 

 strength of those currents. When using a thermal too small in diam- 

 eter for him to turn in, the buzzard employs the same tactics. In the 

 part of his spiral that is out of the current he moves rapidly. Wlien 

 he strikes the rising air he pulls up sharply, so sharply, in fact, that 

 the end flight feathers of his wings open like the slots of a "fool- 

 proof" airplane to prevent a stall and loss of control. Watching a 

 buzzard will give evidence that he has a knowledge of, and is using, 

 moving air. 



It is certain that, in the daily life of soaring birds, wind is a neces- 

 sity. They must have rising air. Without it they are all but helpless. 

 Their large, high-lift wings make them excellent sailplanes and ex- 

 ceedingly poor flyers. A kingbird can make life miserable for them. 

 A difference in wings gives the advantage to the smaller bird. 



It is the way in which he is designed that will govern how much or 

 how little the bird can use the wind. Wing loading, or the ratio be- 

 tween wing area and weight ; aspect ratio, or the relationship between 

 length and breadth ; camber, the thickness of the wings ; dihedral, the 

 angle between the wings; and the other problems that beset the air- 

 plane designer, must surely control the efficiency of the bird. If he 

 can make but little use of the wind his range will be restricted. If he 

 can remain aloft with only slight effort he will roam like the wind 

 from one far corner of the globe to the other. 



A quail takes off and climbs with the speed and steepness of a pur- 

 suit plane but he cannot long maintain the great amount of energy 

 which this entails. He gains altitude rapidly and then glides, prefer- 

 ably downhill or downwind. This necessarily confines the quail to 

 short flights and he does not wander far from his birthplace. His 

 neighbor, the dove, climbs at a lesser angle and uses less energy. But 



