Sense of Hearing, Size, Plumage, Coloration, and Toes of Owls
Except the two main divisions already mentioned, any further arrangement of the Owl must at present be deemed tentative, for the ordinary external characters, to which most systematists trust, are useless if not misleading. (Footnote 89-3) Several systematizers have tried to draw characters from the orifice or the ear, and the parts about it; but hitherto there have not been sufficiently studied to make the attempts very successful. If it be true that the predominant organ in any group of animals furnishes for that group the best distinctive characters, we may have some hope of future attempts in this direction, (Footnote 89-4) for we know that few birds have the sense of hearing so highly developed as the Owls, and also that the external ear varies considerably in form in several of the genera which have been examined. Thus in Surnia, the Hawk-Owl, and in Nyctea, the Snowy Owl, the external ear is simple in form, and, though proportionally larger than in most birds, it possesses to very remarkable peculiarities, -- a fact which may be correlated with the diurnal habits of these Owls -- natives of the far north, where the summer is season of constant daylight, and to effect the capture of prey the eyes are perhaps more employed than the ears. (Footnote 89-5) In Bubo, the Eagle-Owl though certainly more nocturnal in habit, the external ear, however, has no very remarkable development of conch, which may perhaps be accounted for by the ordinary prey of the bird being the larger rodents, that their size are more readily seen, and hence the growth of the bird’s auditory organs has not been much stimulated. In Strix (as the name is here used), a form depending greatly on its sense of hearing for the capture of its prey, the ear-conch is much enlarged, and it has, moreover, an elevated flap or operculum. In Asio, containing the Long-eared and Short-eared Owls of Europe, Asia, and America, the conch is enormously exaggerated, extending in a semicircular direction from the base of the lower mandible to above the middle of the eye, and is furnished in its whole length with an operculum. (Footnote 90-1) But what is more extraordinary in this genus is that the entrance to the ear is asymmetrical—the orifice on one side opening downwards and on the other upwards. This curious adaptation is carried still further in the genus Nyctala, containing two or three small pieces of the Northern hemisphere, in which the asymmetry that in Asio is only skin-deep extends, in a manner very surprising, to several of the bones of the head, as may be seen in the Zoological Society’s Proceedings (1871, pp. 739-743), and in the large series of figures given by Messrs Baird, Brewer and Ridgway (N. Am. Birds, iii. pp. 97-102).
Among Owls are found birds which vary in length from 5 inches -- as Glaucidium cobanense, which is therefore much smaller than a Skylark -- to more than 2 feet, a size that is attained by a many species. Their plumage, none of the feathers of which possesses an aftershaft, is of the softest kind, rendering their flight almost noiseless. But one of the most characteristic features of this whole group in the ruff, consisting of several rows of small and much-curved feathers with stiff shafts -- originating from a fold of the skin, which begins on each side of the base of the beak, runs above the eyes, and passing downwards round and behind the ears turns forward, and ends at the chin -- and serving to support the longer feathers of the "disk" or space immediately around the eyes, which extend over it.
A considerable number of species of Owls, belonging to various genera, and natives of countries most widely separated, are remarkable for exhibiting two phases of coloration -- one in which the prevalent browns have a more or less rusty-red tinge, and the other in which they incline to grey.
Another characteristics of nearly all Owls is the reversible property of their outer toes, which are not unfrequently turned at the bird’s pleasure quite backwards. Many forms have the legs and toes thickly clothed to the very claws; other have the toes, and even the tarsi, bare, or only sparsely beset by bristles. Among the bare-legged Owls those of the Indian Ketupa are conspicuous, and these feature is usually correlated with their fish-catching habits; but certainly other Owls that are not known to catch fish present much the same character.
FOOTNOTES
(89-3) It is very much to be regretted that a very interesting form of Owl, Sccloglaux albifacies, peculiar to New Zealand, should be rapidly becoming extinct, without any effort, so far as is known, being made to ascertain its affinities. It would seem to belong to the Strigine section, and is remarkable for its very massive clavicles, that unite by a kind of false joint, which is some examples may possibly be wholly anycylosed, in the median line.
(89-4) This hope is strengthened by the very praiseworthy essay on the Owls of Norway by Herr Collett in the Forhandlinger of Christiania for 1881.
(89-5) But this hypothesis must not be too strongly urged; for in Carine, a more southern for of nocturnal (or at least crepuscular) habits, the external ear is perhaps even more normal. Of course by the ear the real organ of hearing is here meant, not to tuft of feathers often so called in speaking of Owls.
(90-1) Figures of these different forms are given by Macgillivray (Brit. Birds, iii. pp. 396, 403, and 427).
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