The nearest ally of this species is the B. ferox, of the Palæarctic Realm (Northern Asia and Africa and portions of Europe), which has exactly the size and proportions of the present bird, and in certain stages a very similar plumage. I have not seen an unquestionable adult of B. ferox, but specimens almost adult, in the collection of the Boston Society of Natural History, from the Himalaya Mountains, come remarkably close to B. cooperi in plumage, having like it a black spot on the under side of the wing, but apparently on the under primary-coverts, instead of on the lining, near the edge; the tail is also very similarly colored. Upon the whole, I consider the B. cooperi to be a good species, with B. ferox, Gmelin, of Asia, etc., as its nearest relative, unless it proves to be a hybrid between Buteo borealis and Archibuteo ferrugineus, which I think is less likely to be the case.
Habits. A single individual of this bird was shot by Dr. Cooper near Mountain View in Santa Clara Valley, California, in November, 1855. It still remains unique in collections, and during his more recent explorations Dr. Cooper has not been able to obtain any additional specimens or see any like it. Those he mistook for this bird and to which he refers in his report on the birds of Washington Territory, he is satisfied were only the Archibuteo ferrugineus. The suggestion of Sclater, that the bird is not distinct from Buteo erythronotus, is negatived, according to Mr. Ridgway, by the fact of their actually belonging to different sections of the genus.
Genus ARCHIBUTEO, Brehm.
- Archibuteo, Brehm, 1828. (Type, Falco lagopus, Gmelin.)
- Triorchis, Kaup, 1829 (nec. Leach, 1816). (Same type.)
- Butaëtes, Less. 1831. (Same type.)
- ? Butaquila, Hodgs. 1844. (Type, Butaquila strophiata, Hodgs.)
- ? Hemiaëtus, Hodgs. 1844. (Same type.)
Char. Similar to Buteo, but bill and feet weaker, wings longer, and tarsi feathers in front, to the toes. Bill small, compressed anteriorly, but very broad through the gape; upper outline of the cere ascending basally; nostril broadly oval, nearly horizontal. Tarsus densely feathered in front and on the sides down to the base of the toes; naked behind, where covered with irregular scales. Tarsus more than twice as long as the middle toe; basal half of the toes covered with small scales; outer toe longer than the inner; claws long, strongly curved, acute. Feathering of the head and neck normal. Wing very long; the third to fourth quill longest; first shorter than seventh; outer four or five with inner webs deeply emarginated. Tail moderate, rounded. Plumage full and soft.
The relationship of this well-marked genus appears to be nearest to Buteo and Circus, with an approach to Circætus in character of the plumage, especially the wing. The Old World species, belonging to the subgenus (?) Butaquila, numbering two or three, according to different authors, I have not seen, and consequently cannot say whether they are really congeneric with the American species or not. Exclusive of these, two species are known, both of which belong to North America, one of them (A. lagopus) being found also in Europe and Africa. These differ very considerably from each other, in the details of external structure, probably quite as much as they do from the Asiatic forms above mentioned. The following synopsis will express the differences between the two North American species, and between the American and European races of the one common to both continents.
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