THE AERIAL ENVIRONMENT 



not of velocity, some recognizable heavenly body or bodies. It may also be 

 presumed that it usually has such awareness as a result of seeing the surface 

 of the earth." Acworth (1946:28) says that to a bird in flight "there is no 

 such thing as 'wind/ the bird being, in fact, in a dead calm so far as pressure 

 is concerned." 



This is the stand of airmen, based not on theory, but on the personal 

 experience of flight. "The space in which the pilot flies is not the abstract 

 space of theories, nor the lines and figures of the stereoscope, nor the space 

 of the usual laboratory apparatus for studying depth perception. It does 

 not consist of objects at varying empty distances. It consists chiefly of one 

 basic object, a continuous surface of fundamental importance — the ground. 

 A pilot who cannot see the ground or sea is apt to lose touch with reality in 

 his flying" (Gibson, 1950:59). Without the aid of instruments and without 

 visual reference to the earth, a pilot is unable to judge the direction or the 

 force of the wind and quickly loses geographical orientation. A complaint 

 might be placed against this analogy on the ground that the airman is en- 

 closed within his craft, protected from the air. The objection, however, does 

 not hold on a physical basis; the inability of the pilot to judge the wind 

 aloft may be tested in an open cockpit or by having him thrust his arm into 

 the air stream. With a constant air speed, variations in the direction or the 

 velocity of the wind cannot be perceived, regardless of the craft's relation 

 to the wind. 



The speed of the bird's movement over the ground, the ground speed, 

 depends upon the force and direction of the wind relative to the velocity 

 and direction of flight. When a Mallard (or an aircraft) flies 45 miles per 

 hour in still air, its ground speed and its air speed are exactly the same. Its 

 ground speed is reduced when it moves against the wind, so that with a 

 45 m.p.h. air speed, the ground speed is 30 m.p.h. when the bird flies di- 

 rectly into a 15 m.p.h. wind. Against a 30 m.p.h. wind its ground speed is 

 reduced to 15 m.p.h. With a 15 m.p.h. wind directly on its tail, this Mallard's 

 ground speed is increased to 60 m.p.h. Air speeds of 65 to 70 m.p.h. have 

 been recorded for ducks (Cooke, 1937) ; and with tail winds of 30 to 40 

 m.p.h., which sometimes occur during periods of late-autumn migration, 

 waterfowl are able to progress over the ground at speeds better than 100 

 m.p.h. as indeed we have observed them at Delta in October and November 

 passages. 



Very often the goal of travel is not precisely with or against the wind, 

 the duck meeting a quartering or beam flow of air. The bird must then com- 



57 



