252 



REW ON THE MECHANISM OF FLIGHT. 



feathers are present, and where, consequently, no opening or closing of them can pos- 

 sibly occur; in some birds, moreover, as the Albatros, which spend the greater part of 

 their time on the wing, the wing-coverts are so numerous and come so low down that 

 the primary and secondary feathers (Plate XV. fig. 64 b, a) are with difficulty thrown 



out of position, this being in a great measure unnecessary 



as the wings are 



almost 



always in the extended state. 



Flexion of the Wing necessary to the Flight of Birds.— Considerable diversity of opinion 

 exist s as to whether birds do or do not flex their wings in flight. The discrepancy is owing 

 to the great difficulty experienced in analyzing animal movements, particularly when, as 

 in the case of the wings, they are consecutive and rapid. My own opinion is, that the 

 wings an' flexed in flight, but that all wings are not flexed to the same extent, and that 

 what holds true of one wing does not necessarily hold true of another. To see the flexing 

 of the wimc properly, the observer should be either immediately above the bird or directly 

 beneath it. If the bird be contemplated from before, behind, or from the side, the up 

 and down strokes of the pinion distract the attention and complicate the movement to 

 sucli an extent as to render the observation of little value. In watching rooks proceed- 

 ing leisurely against a slight breeze, I have over and over again satisfied myself that the 

 wings are floxed during the up stroke, the mere extension and flexion, with very little 

 of a down stroke, in such instances sufficing for propulsion. I have also observed it in 

 the Pi -von in full flight, and likewise in the Starling, Sparrow, and Kingfisher. 



Diagram 17. 



Show, the upward inclination of the body end the flesed condition of the wi, lga (a J, , f) m the m , ht of the K i„gfl,her. 



It seems to occur principally at the wrist-joint, and gives to the win- the peculiar 

 quiver or tremor so apparent in rapid flight, and which is likewise well seen in voung 

 birds at feedmg-time. The object to be attained is manifest. Bv the flexing of the wing 

 in flight, the " rmige," or rowing-feathers, are opened up or thrown out of position, and 

 the wind permitted to escape-advantage being thus taken of the peculiar action of the 

 ..ulmdual feathers and the higher degree of differentiation perceptible in the wing of the 

 bud as compared with that of the bat and insect. 



In order to corroborate the above opinion, I extended the wines of several birds as in 



o 



