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 ex 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. 
