A History of North American Birds, Land Birds. Volume 3. Robert Ridgway
(e.g. Strix and Otus) are most depressed anteriorly, have the orbital septum thicker, the palatines longer and narrower, and a deeper longitudinal median valley on the superior surface, and vice versa.
The following classification is based chiefly upon external characters; but these are in most instances known to be accompanied by osteological peculiarities, which point to nearly the same arrangement. It is intended merely as an artificial table of the North American genera, and may be subjected to considerable modification in its plan if exotic genera are introduced.8
Genera and Subgenera
A. Inner toe equal to the middle in length; inner edge of middle claw pectinated. First quill longer than the third; all the quills with their inner webs entire, or without emargination. Tail emarginated. Feathers of the posterior face of the tarsus recurved, or pointed upwards.
1. Strix. No ear-tufts; bill light-colored; eyes black; tarsus nearly twice as long as middle toe; toes scantily haired. Size medium. Ear-conch nearly as long as the height of the skull, with an anterior operculum for only a portion of its length; symmetrical.
B. Inner toe decidedly or much shorter than the middle; inner edge of middle claw not pectinated. First quill shorter than the third; one to six outer quills with their inner webs emarginated. Tail rounded. Feathers of the posterior face of the tarsus not recurved but pointed downwards.
I. Nostril open, oval, situated in the anterior edge of the cere, which is not inflated.
a. Cere, on top, equal to, or exceeding, the chord of the culmen; much arched. Ear-conch nearly as long as the height of the skull, with the operculum extending its full length; asymmetrical.
2. Otus. One or two outer quills with their inner webs emarginated. With or without ear-tufts. Bill blackish; iris yellow. Size medium.
Ear-tufts well developed; only one quill emarginated … Otus.
Ear-tufts rudimentary; two quills emarginated … Brachyotus.
b. Cere, on top, less than the chord of the culmen; gradually ascending basally, or level (not arched). Ear-conch nearly the height of the skull, with the operculum extending only a part of its full length, or wanting entirely.
† Anterior edge of the ear-conch with an operculum; the two ears asymmetrical.
3. Syrnium. Five to six outer quills with their inner webs emarginated. Top of cere more than half the culmen. Without ear-tufts. Bill yellow; iris yellow or black. Size medium or large.
Six quills emarginated; toes densely feathered, the terminal scutellæ concealed; iris yellow. Size very large … Scotiaptex.
Five quills emarginated; toes scantly feathered, the terminal scutellæ exposed; iris black. Size medium … Syrnium.
4. Nyctale. Two outer quills with inner webs emarginated. Top of cere less than half the culmen, level. Without ear-tufts. Bill yellow or blackish; iris yellow. Size small.
†† Anterior edge of the ear-conch without an operculum. The two ears symmetrical. Tail slightly rounded, only about half as long as the wing.
5. Scops. Two to five quills with inner webs emarginated; second to fifth longest. Bill weak, light-colored. Ear-conch elliptical, about one-third the height of the head, with a slightly elevated fringed anterior margin. Size small; ear-tufts usually well developed, sometimes rudimentary.
6. Bubo. Two to four outer quills with inner webs emarginated; third to fourth longest. Bill robust, black. Ear-conch elliptical, simple, from one third to one half the height of the skull. Size large. Ear-tufts well developed or rudimentary.
Ear-tufts well developed. Two to three outer quills with inner webs emarginated; lower tail-coverts not reaching end of the tail. Toes covered with short feathers, the claws exposed, and bill not concealed by the loral feathers … Bubo.
Ear-tufts rudimentary. Four outer quills with their inner webs emarginated; lower tail-coverts reaching end of the tail. Toes covered with long feathers, which hide the claws, and bill nearly concealed by the loral feathers … Nyctea.
††† Similar to the last, but the tail graduated, nearly equal to the wing.
7. Surnia. Four outer quills with inner webs emarginated. Third quill longest. Bill strong, yellow; ear-conch simple, oval, less than the diameter of the eye. Size medium; no ear-tufts.
II. Nostril, a small circular opening into the surrounding inflated membrane of the cere. Ear-conch small, simple, oval, or nearly round, without an operculum.
First quill shorter than the tenth.
8. Glaucidium. Third to fourth quills longest; four emarginated on inner webs. Tarsus about equal to the middle toe, densely feathered. Tail much more than half the wing, rounded. Bill and iris yellow. Size very small.
9. Micrathene. Fourth quill longest; four emarginated on inner webs. Tarsus a little longer than middle toe, scantily haired. Tail less than half the wing, even. Bill light (greenish ?); iris yellow. Size very small.
First quill longer than sixth.
10. Speotyto. Second to fourth quills longest; three emarginated on inner webs. Tarsus more than twice as long as middle toe, closely feathered in front to the toes, naked behind. Tail less than half the wing, slightly rounded. Bill yellowish; iris yellow. Size small.
In their distribution, the Owls, as a family, are cosmopolitan, and most of the genera are found on both hemispheres. All the northern genera (Nyctea, Surnia, Nyctale, and Scotiaptex), and the majority of their species, are circumpolar. The genus Glaucidium is most largely developed within the tropics, and has numerous species in both hemispheres. Otus brachyotus and Strix flammea are the only two species which are found all over the world,—the former, however, being apparently absent in Australia. Gymnoglaux, Speotyto, Micrathene, and Lophostrix are about the only well-characterized genera peculiar to America. Athene, Ketupa, and Phodilus are peculiar to the Old World. The approximate number of known species (see Gray’s Hand List of Birds, I, 1869) is about two hundred, of which two, as stated, are cosmopolitan; six others (Surnia ulula, Nyctea scandiaca, Glaucidium passerinum, Syrnium cinereum, Otus vulgaris, and Nyctale tengmalmi) are found in both halves of the Northern Hemisphere; of the remainder there are about an equal number peculiar to America and the Old World.
As regards the distribution of the Owls in the Nearctic Realm, a prominent feature is the number of the species (eighteen, not including races) belonging to it, of which six (Micrathene whitneyi, Nyctale acadica, Syrnium nebulosum, S. occidentale, Scops asio, and S. flammeola) are found nowhere else. Speotyto cunicularia and Bubo virginianus are peculiarly American species found both north and south of the equator, but in the two regions represented by different geographical races. Glaucidium ferrugineum and G. infuscatum (var. gnoma) are tropical species which overreach the bounds of the Neotropical Realm,—the former extending into the United States, the latter reaching to, and probably also within, our borders. Of the eighteen North American species, about nine, or one half (Strix flammea var. pratincola, Otus brachyotus, O. vulgaris var. wilsonianus, Syrnium cinereum, Nyctale acadica, Bubo virginianus, and Scops asio, with certainty, and Nyctea scandiaca var. arctica, and Surnia ulula var. hudsonia, in all probability), are found entirely across the continent. Nyctale tengmalmi, var. richardsoni, and Syrnium nebulosum, appear to be peculiar to the eastern portion,—the former to the northern regions, the latter to the southern. Athene cunicularia var. hypugaea, Micrathene whitneyi, Glaucidium passerinum var. californicum, Syrnium occidentale, and Scops flammeola, are exclusively western, all belonging to the southern portion of the Middle Province and Rocky Mountain region, and
My unpublished determinations of the North American species were furnished, by request, to Dr. Coues, for introduction into his “Key of North American Birds”; consequently the names used in these pages are essentially the same as those there employed.