262 NATURAL HISTORY OF BIRDS. 



away from the wind, sweeps with increasing velocity around another incomplete 

 circle, and towards its close rises another step on his airy staircase. 



Sometimes, impatient apparently of the slight gain at each turn, a circle will be 

 finished with a vigorous flap or two in the face of the wind, and then the steady 

 circling motion be resumed again. 



Of course the lower currents of air may often have a different direction or velocity 

 from the upper ones, and this doubtless often enables the soaring bird to retrieve much 

 of his loss due to lateral drift, or even sometimes to pass readily in what seems to be 

 the very wind's eye. Undoubtedly, if the bird were willing and able to keep its 

 wings inclined at the proper angle to the breeze without circling, it could rise in much 

 the same manner as a paper kite, drifting, however, rapidly to leeward, but it would not 

 be an economical use of either time or strength. The rapid wheeling in wide curves 

 gives a bird complete control of its wings and enables it to preserve its balance per- 

 fectly without a thought, and so take advantage of every breath that blows. One 

 need only reflect for a moment how much easier it is to roll a plate or wheel in a 

 straight line on its edge than to balance it there when at rest, in order to realize the 

 advantage gained through the continuous and rapid motion. 



It is obvious also that in two birds of different weights, but with relatively the 

 same wing-surface, the heavier bird will have the advantage over the lighter, through 

 the greater steadiness which the greater weight insures. I do not mean to say that 

 only the large birds of prey vise in this way. The pigeon-hawk, Falco columbarhis, 

 and many other small falcons, avail themselves of this power, but it is certainly seen 

 on a grander scale, if not in higher perfection, among the eagles, vultures, and largest 

 falcons. This is not the place to enlarge on such a subject, but it is introduced be- 

 cause so many of the birds of prey are experts in this kind of flight, which is less 

 commonly understood, perhaps, than the more usual mode of progression by vigorous 

 wing-beats. 



It only remains to say that probably our utmost stretch of imagination does not 

 enable us properly to conceive how slight is the loss resulting from friction of the air, 

 and at the same time how great the lifting power of an almost imperceptible breeze 

 on a bird moving rapidly against it. 



In general, the food of all members of the order consists of the flesh of vertebrates, 

 the exceptions being comparatively few, and mostly in the direction of insect-feeding. 

 Of the forms which subsist on living prey, the great majority probably feed on other 

 birds, to which diet is added a liberal proportion of mammals. A much smaller frac- 

 tion of the whole draws the bulk of its supplies from the reptiles, while fewer members 

 still feed largely on fish. This latter fact is rather surprising, considering the abun- 

 dance and accessibility of such food. 



The influence which the Accipitres exert among birds and other vertebrates is an 

 extremely salutary one, though we lack the data for determining to what extent they 

 aid in holding noxious forms in check, it being very certain that many of them are 

 indiscriminate feeders, capturing beneficial as often as harmful species. It is ex- 

 tremely improbable, however, that they have ever been instrumental in the entire 

 extinction of species, and in most cases it may well be questioned whether the judi- 

 cious weeding out of weakly forms is not one of their most important benefactions. 



In the taking and killing of prey it is interesting to note that the feet are invari- 

 ably the efficient Aveapons, the beak being rarely used until after the victim is dead. 

 Even the insect-eating kites and falcons clutch their minute prey with their feet, after- 



