PETTIGREW ON THE MECHANISM OF FLIGHT. 2G1 



ordinary purposes of flight. This is especially the case if the bird is advancing against 



slight breeze, the effort required under such circumstances beiim- nominal in am 



. T 



little power is expended in such instances is proved by the endless gyrations of Rooks and 

 other birds, these being continued for hours together. In birds which glide or skim, it has 



appeared to me that the wing is recovered much more quickly and the downward stroke 

 delivered more slowly than in ordinary flight — in fact, that the rapidity with which the 

 wing acts in an upward and downward direction is, in some instances, nu >re or les^ reversed ; 

 and this is what we should naturally expect if we recollect that in gliding the winL - 



in gliding the wings require 

 to be, for the most part, in the expanded condition. If this observation be correct, it fol- 

 lows that birds have the power of modifying the duration of the up and down stroke at plea- 

 sure. Although the wing of the bird usually strikes the air at an angle of 30° or so, t hat 

 angle may be increased to such an extent as to assist in reversing the position of the bird, 

 as happens in the Tumbler Pigeon, which can, by slewing its wings forwards and suddenly 

 throwing back its head, turn a somersault. When birds are fairly on the wing they have 

 the air, unless when that is greatly agitated by a storm, completely under control. This 

 arises from their greater specific gravity, and because they are possessed of i ndependent mo- 

 tion. If they want to turn, they have simply to tilt their bodies laterally, as a railway car- 

 riage would be tilted in taking a curve*, or to increase the number of beats given by the one 



t as compared with the other, or to keep the one wing extended while the other is 

 partially flexed. The neck, feet, and tail may or may not contribute to this result \. 

 If the bird wishes to rise, it tilts its entire body (the neck and tail participating 

 upward direction (Diagram 17, p. 252); or it rises principally by the action of the 



* Captaiu F. W. Hutton, in speaking of the Albatros, says, "If he wishes to turn to the right he bends his head 

 and tail slightly upwards, at the same time raising his left side and wing, and lowering the right in proportion to the 

 sharpness of the curve he wishes to make, the wings being kept quite rigid the whole time. To such an extent does 

 he do this, that, in sweeping round, his wings are often pointed in a direction nearly perpendicular to the sea ; and this 

 position of the wings, more or less inclined to the horizon, is seen always and only when the bird is turning."— On 



■-i 



me of the Birds inhabiting the Southern 

 t " The turning in flight to the right or 



quality 



»/ 



To wheel to the right, the left wing must be plied with greater frequency or force, and vice ver«tr—Comp. Anat. $r 



Mr. Macgillivray, as will be seen from the following 

 passage," hold an opposite opinion. " "Birds in flying always beat their wings synchronously, never alternately, 

 even in executing rapid turns, which are effected by the contraction of the one wing and the extension of the 

 other, aided by the tail r -Macgillivray, I c vol. i. p. 420. The Duke of Argyll does «* **^J*^ 

 like action of the tail, 

 set, not as it is, horizontally, but perpendicularly 



d states that, - if the tail had been intended as a rudder, its vane would hare been 



• - .„ „„„Wally, hut perpendicularly to the line of flight."-' Good »"W, for rebruary 18*5, 



P- 132. Although the tail of the bird is expanded horizontally or laterally, and uot vert.eally, as .s the rudde, 

 » ought not to be overlooked that the muscles of the tail are arranged iu such a manner as to adnut of _«» bemg 



•ilted m either side> M h in the tail of the W hale, Dugong, Manatee, Porpo ise, and Beaver and that J hen 



30 tilted it materially assists iu wheeling. Further, it does not follow, because a b,rd can fly ™ bout a tad o 

 ™h a very small one , that the evo i utions of th e bird are not facilitated by its presence On the contran , ,b bmfc 



perf 



sion of their movements (say the birds of prey) are usua 

 an organ, moreover, is no proof of its utility when presen 



-- -*vn tins urgaii in penecuon. me auseuuc ui «** ^*& — > — - - ,, ;l t u m 



■t The tad , 6wlicd „ lth muscles t0 spread the feathers and to £~££~ £ ^ »J^ 



obhqnely. " The taU-fcathers may be extremely short, short, moderate, long extremely lou D , a ^ 



gradation,, even, rounded, graduated, or wedge-shaped at the end, and tapenng, po.nted, emargmate or forked. 



Macyill 



VOL. XXVI. 



2 



