250 THE STRUCTURE AND LIFE OF BIRDS chap. 



change of balance is effected the difficulty begins. 

 We might naturally suppose that one wing beats 

 harder than the other ; in the turn to the left the 

 right wing would work with greater energy, so as to 

 throw the body on to its left side. That being so, it 

 is puzzling at first, when we see the turn made, to 

 find that the right wing which should give the harder 

 stroke is held aloft while the left wing is lowered. 

 This, however, is not an insuperable difficulty. Each 

 wing makes the same angle with the body, so that a 

 line connecting the tips would either pass through the 

 shoulder-joints or be parallel to a line passing through 

 them. It is only the rolling over of the body on to 

 its side that causes them to point upward and down- 

 ward respectively. This being the position in which 

 we see the wings as the bird wheels, can the right 

 have struck harder than the left in order to effect the 

 turn? Possibly it may have. In ordinary flight, as 

 we have seen, the body at once rises in response to 

 the beat of the two wings together, and thus they are 

 brought home to their lower limit, without the stroke 

 having to be pulled through. The right wing, then, 

 if it gave the stronger beat, might instantaneously 

 raise the body on the right side and produce the 

 desired result, the change of balance ; when, very 

 possibly, all we should see might be the bird sailing 

 on with the right wing pointing upward and the left 

 downward. Still there would be an instant at which 

 the wings would form unequal angles with the body, 

 and what evidence of it can we obtain ? If a Gull be 

 watched as he slowly wheels, there seems to be ab- 

 solutely no inequality of angle ; when a rapid turn 



