The Wing of a bird will be found to present a structure worthy of our highest admiration. The object aimed at in the arrangement of its different parts is evidently to obtain a very large and firm expanse of surface, by employing the smallest possible quantity of material. For the purposes of flight it is obviously necessary that the superficial extent of the wings shall be sufficiently ample, not only to sustain the weight of their possessor in the thin and yielding element in which it flies, but, by the vigour of their stroke and the violence of their impulse, to propel the bird with a rapidity proportioned to the occasion, and, moreover, by the lightness of their touch and the accuracy of their movements, to steer and steady its course through all the varied evolutions whereby it is enabled to capture prey, or sportively display the wonderful activity conferred upon the feathered tribes.
We have already seen, while examining the construction of the skeleton, that the bony framework of the wing essentially resembles that of the human arm, and that the limb when stripped of its feathers is no more adapted for flying than our own. The needful expansion is obtained altogether by the addition of the quill feathers, which, as explained in a preceding page, combine in their structure all the qualities requisite for the intended purpose, lightness, firmness, strength, elasticity, and extent of surface: the central part of the arm or wing forms merely a basis of support, into which the quills and other appendages to the wing are securely implanted. It will therefore be easily understood that the importance of the individual quills as instruments of flight will depend very much upon the position they hold in the wing, of which they form so considerable a portion, and, consequently, that they have received names expressive of their relative efficiency. Those that are affixed to the bones representing the hand (Fig. 12, p, q q), by the length of their stroke, and the peculiarity of their arrangement, are obviously of primary importance, both from their size and the situation they occupy, and have consequently been named the "primaries," or the "primary quills," (Figs. 2, f; 13, 35); they might be called with equal propriety the "hand-quills," a term more particularly expressive of the parts to which they are attached. Upon the relative length and other proportions of the primary quills the shape and mechanical power of the instrument principally depend; if the first primary be the longest, the termination of the wing is sharp and pointed (acuminate), as in most birds that are remarkable for the swiftness of their flight; whereas if the second, third, or fourth of these quills should exceed the others in this respect, the wing becomes more and more rounded (obtuse), and the perfection of its action visibly deteriorated.
The "Secondary Quills," or "Secondaries" (Figs. 2, c; 13, 34), are exclusively sustained by the bones of the fore-arm (Fig. 12, m, n); from their situation being much nearer to the shoulder-joint than the preceding, the extent of their sweep is more limited, and their stroke much feebler; they are, consequently, as their name indicates, of secondary importance in locomotion.
Plate 4 Cassell's Book of Birds
1. Tyrant Shrike (Tyrannus intrepidus).—2. Virginian Chordeiles (Chordeiles Virginianus).—3. Cow-bird (Icterus pecoris).—4. Crimson Honeysucker (Myzantha garrula).—5. Masked Weaver-bird (Ploceus larvatus).—6. Fire-crested Wren (Regulus pyrocephalus).—7. Rice-bird (Padda oryzivora).—8. Variegated Grakle (Quiscalus versicolor).—9. White-tailed wheatear (Saxicola leucura).—10. The Wittal Oriole (Oriolus galbula).—11. The Alpine Accentor (Accentor Alpinus).—12. Nordman's Glareole (Glareola Nordmanni).—13. Alpine Tit (Parus Alpinus).—14. Crested Tit (Parus Cristatus).—15. African Parra (Parra Africana).—16. Piririguan Tick-eater (Crotophaga Piririgua).—17. Ptarmigan (Lagopus Scoticus).—18. Scapulated Crow (Corvus Scapulatus).—19. The Kestrel (Tinnunculus Cenchris).—20. The Senegal Thick-knee (Oedicnemus Senegalensis).—21. Rock Kestrel (Tinnunculus rupicolus).—22. Corythus (Corythus enucleator).—23. The Red-legged Stilt-Walker (Himantopus rufipes).—24. The Stone Thrush (Turdus saxatilis).—25. The Silk-tail (Bombycilla garrula).—26. Long-tailed Tit (Parus biarmicus).—27. The Wall-creeper (Tichodroma muraria).—28. The Troopial (Agelaius phœniceus).—29. The Yellow Bunting (Emberiza Cia).—30. The Red-eyed Coccyzus (Coccyzus erythropthalmus).—31. The Cirl-Bunting (Emberiza cirlus).]
The Spurious, or Bastard Quills (Figs. 2, e; 13, 33), are attached to the rudimental bone that represents the thumb (Figs. 2, g; 12, p); their size is diminutive, and their use in flight comparatively unimportant.
The Wing Covers or Coverts (Fig. 13, 37) are small feathers arranged in several rows, which overlap and strengthen the bases of the quills; they are often variously coloured, and thus afford important features whereby different species may be distinguished. Besides the above, there are certain conventional terms employed by the ornithologist that will require enumeration. The arm-part of the wing in the living bird is generally known as the shoulder (27); the elbow-joint is the flexum; while that part of the fore-arm which corresponds to the edge of the wing is denominated the shoulder-margin (6).