POSITIONS ASSUMED BY BIRDS IN FLIGHT. 203 



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 Avill compensate for the current. This is 

 suggested in Figure 6, where the four central birds are 





Pter- ^ 



•%► /T- 



FIGURE 6. — SUDDENLY AND HARMONIOUSLY WHEELING. 

 (Photographed by Bentley Beetham.) 



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. 



