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 clipped 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 alighted 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 gulls afford instances 
of this, when, during windy weather, or for other reasons, 
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