Figure 5. The "calculated course." The bird travels along the shore line, but to hold 



this course it must face away from the line of flight into the wind. The slower 



its air speed, the more nearly the bird must face into the wind. 



pensate for the angular force of the wind to obtain its objective. When 

 ducks fly westward along the lakeshore, for example, and the wind is blow- 

 ing from the northwest, the birds do not point like arrows in the direction 

 of their movement over the ground; their bodies aim into the northwest, 

 away from the shore and into the wind. Airmen call this "crabbing," a com- 

 mon and essential technique in human flight. The course of the bird is set 

 by its eyes, thus demanding the bodily adjustment to the force of the wind. 

 The degree to which the bird must face into the wind depends on the air 

 speed of the bird and the velocity of the wind according to the "triangle 

 of velocities" (Allen, 1939; Smith, 1945; Jack, 1953) (Figure 5). This pat- 

 tern of flight is seen most frequently where waterfowl follow a shoreline or 

 a river or stream, but I have seen high-flying stubble Mallards "crabbing" 

 on an overland route to the fields. 



Another technique for adjusting to the wind is the "duffer's" course 

 (Smith, 1945:155), where the bird aims its body continually at its destina- 

 tion without regard to the overland path required to reach its objective 

 (Figure 6). This technique is most frequently observed where birds are 

 moving short distances and where the terrain is very broken. Smith (1945) 

 speaks of starlings flying in a series of duffer's curves from one landmark 



58 



