ON SOARING FLIGHT. 201 



that being no longer supported at that point, tbey sought the spot 

 from whence the first flock was to be seen rising; while the fact that 

 each bird was successively buoyed up on reaching the spot indicated 

 the existence of a well-established ascending current which extended 

 to a height of 1,000 feet or more. 



Again on a clear day with light winds a flock of forty vultures was 

 frightened from a carcass on a summit of a ridge. They flew directly 

 down a valley in a direct line with much flapping, a distance of a half 

 mile and with a common impulse stopped and began flying in circles 

 in a somewhat compact mass. Xot one of the entire flock succeeded 

 in soaring without flapping although they continued to make the 

 attempt for two minutes. Evidently there was no rising current at 

 that point and they did not succeed in making one. But it is signifi- 

 cant that they should have selected a spot in which to soar where no 

 current existed, for it weakens the assumption which might be made 

 that the birds begin soaring in circles only when they encounter rising 

 currents, and that they have some delicate sense by means of which 

 they are able to detect them and find their way into them. At the end 

 of two minutes the flock divided into two. The one moved a few hun- 

 dred feet to the north over an open field and after some preliminary 

 flapping soon began soaring; the other moved south and attempted to 

 soar above a wooded slope on a north hillside, but failing in the 

 attempt a large x>ortion abandoned the spot and sailed away to join 

 the first flock where they at once began soaring with the others. Those 

 which remained, not being able to soar, finally alighted on some trees 

 and remained there. Here again the birds which flew south endeav- 

 ored to soar over a spot where, as the event proved, no rising current 

 existed, and this a part of them continued to do in the api^arent expec- 

 tation that one would rise if they continued to circle long enough, their 

 instincts not teaching them that the cold earth underneath them did 

 not favor the production of such a current. But the greater part, see- 

 ing that the other flock had began soaring, set off at once to join them, 

 and the fact that when they had done so they also at once began soar- 

 ing shows clearly that, whether the first flock found it or generated it, 

 they were borne up by a rising current, and that it is the condition of 

 the air and not the form of the wing only which enables the bird to 

 soar. That the birds are borne up by ascending currents, either natur- 

 ally or artificially produced, is indicated by the not unusual sight of a 

 bird passing in a direct line beneath another soaring in circles, the former 

 rising and falling precisely as it enters and leaves the current. 



Again, I once saw a half dozen vultures try in vain to soar above a 

 carcass on the summit of a wooded hill. "Wearying of the attempt, 

 three of tliem deliberately set sail to a spot over a valley a few hundred 

 feet distant, and with a common impulse began flying in circles. After 

 a short time rhey began rising, but only rose to a height sufficient for 

 their purpose, Avhen they one after another set sail for the hill again. 



