ON SOAKING FLIGHT. 195 



the trees immediately below. Again, I frightened one from its perch, 

 upon a tree in the edge of a woodland upon the summit of a hill, at a 

 moment when no indication of a wind could be detected in the trees. 

 It at once began soaring, with less than a dozen beats of the wings, 

 circling round 30 feet overhead and slowly rising. I could easily mul- 

 tiply similar instances, in which the bird has been seen soaring in winds 

 so light as to be scarcely discernible and under circumstances which 

 render it wholly improbable that any strong current of air could have 

 escaped detection. 



On the other hand, I have, on numerous occasions, seen the birds 

 attempt soaring and fail, both singly and in companies, when light 

 winds were blowing steadily, especially on dark days and when the 

 earth was cold; nor do I remember at any time to have seen them soar 

 in light winds when the ground was frozen or covered with snow, nor 

 to have seen them attempt it. 



Observation seems to show that the condition which renders soaring 

 flight possible in light winds is that the surface of the earth shall be 

 warmer than the air above it; and this, it will be seen, is the very con- 

 dition which gives rise to light, local, irregular winds. It is not sur- 

 prising, then, that the birds shoald begin soaring with the rising of 

 the winds, nor that observers should have sought for an explanation of 

 their flight in the winds themselves. But it sometimes happens that 

 they begin soaring before any winds can be detected, and as I have 

 endeavored to show, it is this condition of a cold atmosphere above a 

 warm surface resulting in an unstable equilibrium and not the preva- 

 lence of winds that makes soaring possible under these conditions. 

 Moreover, as this condition will ensue whenever the earth is exposed 

 to the heating influence of the sun's rays, and since, if it prevails at all 

 it must prevail over vast regions of the earth's surface, it will be seen 

 that it is neither an abnormal nor a local condition in those countries 

 where the birds are known to habitually soar. This, however, is the 

 condition requisite to flight in light winds only, for observation shows 

 that while the birds may usually soar in light winds, they may invari- 

 ably do so in heavy winds. High winds, therefore, also furnish condi- 

 tions rendering such flight possible. 



But, as already pointed out, there is a marked difference in their 

 manner of soaring under these different circumstances, and it will 

 accordingly be convenient to consider the subject under two divisions — 

 spiral flight in light winds; direct flight in heavy winds — and to inclnde 

 under the latter a supplementary discussion of flight in winds of mod- 

 erate velocity. 



SPIRAL FLIGHT IN LIGHT WINDS. 



If when the winds are light a score of vultures, including turkey 

 buzzards and black vultures, be frightened from a carcass they will 

 immediately take wing, flying near the earth and flapping vigorously. 



