ON SOARING FLIGHT. 199 



still continues in those enormous circlings, those mad wheelings which 

 give one the vertigo merely to watch them. Sometimes those nearest, 

 not quite sure of perfect safety, return upon an upward glide, and | 

 thus a broad, horizontal layer of vultures serves as a base for this 

 interminable whirling column. This continues until the surrounding 

 country has been thoroughly examined and the carcass is deemed acces- 

 sible, when the hungriest dart down upon it." From this it appears 

 that the birds in the act of descending and when restrained through 

 fear have it in their power to stop and remain in the air indefinitely 

 over any given locality. It is a common occurrence for numbers of 

 vultures to be drawn together through the mere chance of obtaining a 

 meal. Thus a single vulture, flying low and inspecting every nook 

 below for prey, pauses to reconnoiter, and begins soaring round and 

 round above some spot. In an incredibly short time the air will be 

 filled with birds, all soaring and intent on finding the supposed car- 

 cass. Failing in this, the search is abandoned. Little by little .the 

 birds collect into a close cluster above the spot which chance alone 

 selected. For a time they scarcely seem to rise at all; then they go 

 higher, suddenly, rapidly, unexpectedly, as if caught upon a rising 

 swell of air. For such occurrences the theory of ascending currents 

 naturally produced fails to satisfactorily account. 



BIRDS OFTEN BEGIN CIRCLING WHERE NO ASCENDING CURRENT 



EXISTS. 



Upon the hypothesis of ascending currents artificially produced by 

 the bird many such phenomena of flight, not otherwise easily under- 

 stood, may be readily explained, as for instance the scarcely perceptible 

 ascent in the early stages of spiral flight, with the subsequent rapid 

 rise. If the birds availed themselves of ascending currents already 

 established they should be found rising as soon as they entered the 

 current; but if the current were produced by their own efforts a cer- 

 tain time would elapse before it attained its full force, and we should 

 find as is actually the case, the birds suddenly and rapidly rising, after 

 a more or less prolonged flight at a fixed elevation. The hypothesis 

 explains also why the birds so often flap vigorously at the beginning 

 of their flight, and, what is significant, why their flapping ceases 

 not gradually but suddenly. It has been suggested that as the bird 

 rises it encounters winds of greater velocity and consequently of greater 

 buoyant power. But spiral flight occurs usually when the winds are 

 either light or absent, and the simultaneous cessation of flapping and 

 the steady rise which follows is commonly attained at low altitudes? 

 nor is there any apparent difference in the character of their flight at 

 the heights of 200 feet and 1,000 feet. Besides high winds enable the 

 bird to soar without circling at all and seem rather to be unfavorable 

 to any great ascent by spiral flight. Incidentally it may be stated that 

 the soaring birds seldom if ever rise to great heights by flapping. But 



