required for the purpose of flight. Dr. J. Bell Pettigrew, 

 more than fifty years ago, to test this matter, cut off more 

 than half of the secondary wing feathers of a sparrow, parallel 

 with the long axis of the wing. He first chpped one, then 

 both wings, and found that in both cases flight was apparently 

 unimpaired. He then removed a fourth of the primary 

 feathers — the outermost quills — and still the flight was 

 unimpaired. At any rate the bird flew upwards of thirty 

 yards, rose to a considerable height and ahghted in a tree. 

 Thirty yards, however, is a short flight even for a sparrow. 

 But it is enough to show that flight, if not sustained flight, 

 was possible after this mutilation. Not until more than one- 

 third of the quills along the whole length of the wing were 

 removed, did the flight become obviously laboured. And 

 he found that what was true of the sparrow, was equally 

 true of the wings of insects. 



Though these experiments demonstrate, in a very unmis- 

 takable manner, that flight with a greatly reduced wing 

 area is possible, we have no evidence that this reduction 

 would make no difference to the length of time the bird could 

 remain on the wing. And this is a very important matter. 



An aspect of flight which has now to be considered is that 

 of birds which fly in troops. Some species always travel thus, 

 others only on occasions. Rooks and guUs afford instances 

 of this, when, during windy weather, or for other reasons. 



47 



