198 ON SOARING FLIGHT. , 



explaining how the bird finds an ascending current is in a measure 

 overcome. Let us now apply this liypothesis to a special case, repre- 

 senting the typical flight of the large squirrel hawks in light local 

 winds. On a clear, calm morning I once frightened two of these hawks 

 from their perches on a dead tree in a level field of perhaps 20 acres in 

 extent. They at once flew away, alternately flai)ping and sailing, and 

 making no effort to keep together. At times they were on opposite 

 sides of the field, each jiursuing its own course, while ranging back and 

 forth. After numerous preliminary attemj)ts the two at length began 

 soaring steadily at a distance apart of several hundred feet. While 

 still circling, and without further flapping, the two gradually approached 

 until the circles of their flight, which had a diameter of perhaps 75 feet, 

 coincided. (One will watch in vain to see these hawks rise to any great 

 height along separated spirals.) After this their flight was steadily 

 ui)ward until they had attained a height so great that they seemed 

 but gray specks against the blue sky. At length they ceased circling 

 and sailed away along separate courses. If we assume that over some 

 spot a strong, steady current was ascending, fed from the stratum of 

 warm air near the earth, and that from all quarters it was slowly being 

 drawn into it, at an average velocity of 2 feet per second, then a period 

 of eight minutes would have sufficed to draw the birds into the cur- 

 rent from a distance of 1,000 feet. It might seem, therefore, that upon 

 this hypothesis we have arrived at a full and satisfactory explanation 

 of soaring flight in light winds. But such is very far from being the 

 case, for while the supposed drifting of the birds into ascending cur- 

 rents doubtless accounts for many of the phenomena of flight, it leaves 

 others wholly unexplained, for while we know but little in regard to 

 ascending currents, either as to their number, dimensions, or duration, 

 it is yet certain that they must be limited in all these respects, and 

 over extended plains the element of chance must be an important 

 factor in their location. Under such circumstances we can not suppose 

 that ascending currents of large magnitude are to be looked for over 

 any given spot where vultures may congregate above a carcass or 

 that such currents are to be found at all hours of the day and for days 

 in succession. Yet the vultures will soar day after day over such a 

 spot. Of their ability to do so at will the following example may be 

 given : Some vultures found the remains of a rabbit which had been 

 rolled in the dirt by a harrow, and others seeing those upon the ground 

 came in large numbers and soared directly over the spot for some min- 

 utes, but, not finding prey, did not come down, and rose instead and 

 sailed away. An hour later there was a repetition of the same per- 

 formance. They came a third time during the day and again on the 

 following day. There was at no time any flapping, there were no wind- 

 breaks to produce ascending currents, and the birds in each instance 

 soared immediately above the same spot. Mouillard thus describes 

 the descent of the Egyptian vultures on the discovery of a carcass: 

 "These myriads of great birds whirl like a water spout. The descent 



