VII 



FLIGHT 231 



material assistance to another following close after. 

 A bird's body, on the contrary, is so shaped that it 

 does not meet with much resistance and consequently 

 does not " break " the air to any great extent, and, in 

 addition to this negative result, the action of his wings 

 causes a back current, to which, in fact, his progress 

 onward is due. A bird who finds himself in this 

 back current is probably at least as badly off as a boat 

 in the wash of another boat. In whatever form birds 

 marshal themselves for flight, their aim probably is to 

 save each from the wash of those in front. Even when 

 they are apparently flying in Indian file, as ducks 

 often do, we need not see an exception to the rule. 

 It is by no means certain that their heads point 

 exactly along the line : very probably all are following 

 parallel tracks a short distance apart (see 3, Fig. 63). 

 It is probable that the formation varies according 

 to the direction of the wind relatively to the line 

 of flight. Unfortunately the wind at a great altitude 

 does not always blow from exactly the same point of 

 the compass as the wind below. Another difficulty 

 is that a flock of birds far above us may be flying 

 some in one horizontal plane, others in another, though 

 they appear to us to be in the same. It is very 

 difficult to make trustworthy observations. 



Changes en route take place frequently. The 

 leader drops behind or there is a general rearrange- 

 ment, apparently quite orderly, each falling without 

 confusion into his place. 



