The Genera of Booted Eagles: Aquila and Relatives*
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J. Yamashina Inst. Ornith., 14: 108-121, 1982 The Genera of Booted Eagles: Aquila and Relatives* Dean Amadon** Abstract The nine genera of eagles with feathered legs (hence "booted") are reviewed. Of the nine, six were monotypic. One of the six, Ictinaetus, is very distinct and is probably not a member of the group. Of the other monotypic genera Polemaetus is here combined with Hieraaetus, and Lophaetus, Oroaetus, and Stephanoaetus with Spizaetus. The remaining mono- typic genus, Spizastur, is for the time being retained because, while very close to Hieraaetus in characters, it may in fact prove to be a New World offshoot of Spizaetus stock. The genus Aquila with 10 species is left as presently constituted. All of the five genera recognized are diagnosed; their way of life briefly characterized, problems at the species level discussed, and the species of each listed in a recommended sequence. Introduction The term "booted" is applied to a group of about thirty species of eagles, so called because their legs (tarsi) are feathered down to the bases of the toes. In most current literature (e.g. Stresemann & Amadon 1979) they are divided among nine genera of which no fewer than six are monotypic; the best known of the genera is Aquila whose type is the well known Golden Eagle, A. chrysaetos. Feathered tarsi occur sparingly elsewhere in the family Accipitridae, for example in the "Rough-legged" Hawk, Buteo lagopus; the condition is of no particular taxonomic significance in and of itself. In the hawk mentioned, and also in such owls as the Snowy (Nyctea scandiaca), not to mention ptarmigan (Lagopus), feathered legs are clearly an adaptation to withstand arctic cold, but in the booted eagles there is no diminution of the feathering in tropical species; conceivably it gives the legs a little additional protection when grappling with dangerous prey. The Indian Black Eagle, Ictinaetus malayensis, which comprises one of the monotypic genera, is a peculiar species in many respects, as detailed later. The genus is clearly valid and may not belong to the same group as the other booted eagles, albeit that the tarsi are feathered, though somewhat thinly. The remaining eight genera, five monotypic, give every appearance of being a monophyletic, closely and intricately interrelated group. A generic reassessment seemed appropriate. In the following analysis each of the genera recognized is followed by a list of the included species. To the extent that it concerns the genus composition, problem taxa- those on the subspecies-species border are discussed. Original citations for species names may be found in Stresemann and Amadon (op. cit.). * Dedicated to my friend Dr . Yoshimaro Yamashina, Pre-eminent Leader of Ornithology in Japan for Half a Century. ** Lamont Curator of Birds , Emeritus; American Museum of Natural History, New York, USA 10024. 108 Genera of Booted Eagles 109 Methods Generic diagnoses for the booted eagles are presented, somewhat informally, in Brown and Amadon (1968). Two of the genera, Aquila and Hieraaetus, are defined in Glutz et al. (1971), based in part on the earlier monograph by Hartert (1913-1914). Hartert also treated Spizaetus, but only briefly and for some reason he separated it from its closest allies. Voous and Wijsman (1964) discussed the generic characters of the booted eagles, especially the African ones. Other references are mentioned below. For only one of the species of the group is the term "booted eagle" used as a vernacular name, viz., Hieraaetus pennatus of Eurasia and Africa. I have twice been responsible for generic treatments of the hawks and eagles so it is reasonable to ask why the changes recommended below were not made previously. Further study in field and museum seemed advisable before doing so. Stresemann and Amadon (1979) is based on an earlier manuscript list by Stresemann and I did not wish to make further reductions in his genera beyond those that seemed essential, as summarized on page 272 of that list. Actually, as there pointed out, Stresemann had himself synonymized one genus of the group under consideration, Lophaetus, a course which I then resisted as it did not seem logical to merge this genus without further consideration of several others. I have observed seven of the nine currently recognized genera in the field; all except Oroaetus and Ictinaetus. Of species, I have observed: Aquila-chrysaetos, verreauxii, rapax, nipalensis, wahlbergi, audax; Spizaetus-ornatus, tyrannus; Hieraaetus-spilogaster, ayresii, morphnoides, and also the single included species of Stephanoaetus, Lophaetus, Spizastur and Polemaetus. It is now conceded that many genera must be defined by a suite of characters not all of which will be found in every included species, although every species will possess a preponderance of them. This is certainly true of the three polytypic core genera of booted eagles Aquila, Hieraaetus, and Spizaetus. They are difficult to diagnose, yet seem to be natural monophyletic units. The paucity of trenchant morphological characters among birds, the result of loss and fusion of skeletal elements, including teeth, is well known. Yet avian genera are often the equivalent of those of other classes of higher vertebrates as regards antiquity, number of included species, and ecological distinctness. Mensural characters: The genera of booted eagles are separated to some extent by relative length of wing, tail and sometimes tarsus, though there are species whose measurements agree with one genus and its other characters with another, e.g., Aquila gurneyi. Wing and tail proportions are here expressed as the ratio of tail to wing as a decimal; for example in most species of Aquila the tail is about half the wing or .50. Obviously this is a very rough measure which may be affected either by a lengthening or shortening of wing or of tail or of both in any particular taxon. Thus in the genus Spizaetus the tail is relatively long and the wings relatively short and rounded to produce proportions like those of a classical "hawk" (Accipiter); the name "hawk-eagle" refers to this. There are correlated changes in the shape of wing and tail: the longer the tail the more apt it is to be rounded or even wedge-shaped; the relatively shorter the wings, the 110 D. Amadon more rounded and broader they are. Ratios of tail to wing given in this paper are based on the wing measured "flat"; the shorter chord measurement, of course, gives a slightly higher ratio. The relative length of the tarsi varies considerably in all the polytypic genera recognized here from species to species. In general, large species that strike heavy, powerful prey have short tarsi and heavy talons. In the African Long-crested Eagle, Spizaetus occipitalis, the legs are especially long, presumably because it strikes at rodents concealed in rank grass. A species' overall or general size is not usually considered a valid generic character in these or other birds. There was already a wide range in size between for example Spizaetus nipalensis orientalis and S. nanus or between Hieraaetus f. fasciatus and H. morphnoides weiskei. Thus adding Stephanoaetus coronatus to the former and Polemaetus bellicosus to the latter genus does not greatly extend the range of variation in size, at least in Spizaetus, where the wing length of coronatus does not much exceed that of the Japanese subspecies of nipalensis, though it is a heavier bird. The measurements used in this paper to calculate ratios were taken directly from specimens or from standard sources such as Brown & Amadon (1968). Synonyms listed under the generic headings include only those having different type species. Relationships of the Booted Eagles to other Eagles The term "eagle" is applied indiscriminately to any especially large hawk. Among other groups so designated are the sea-eagles, the serpent-eagles and the harpy eagles. The booted eagles differ in so many ways from the sea-eagles and the serpent-eagles as to require no detailed comparison with them. On the other hand the resemblance of such a booted eagle as the African Crowned Eagle ("Stephanoaetus") to the Harpy Eagle (Harpia harpyja) in proportions, way of life, and even in the prolongation of the nesting cycle over two years, leads to recurring questions as to whether the two groups may be more closely allied than is generally supposed. The harpy group includes also Morphnus, which is much like Harpia, and two relict Old World genera, Pithecophaga in the Philippines and Harpyopsis in New Guinea. These four monotypic genera comprise a somewhat primitive group, allied with what I call the sub-buteonine hawks and eagles (Brown & Amadon 1968, Amadon 1982). Their long unfeathered coarsely scaled tarsi are only one of several characters separating them from the booted eagles. They are somewhat less advanced, albeit specialized, relatives of the stem genus Buteo, while the booted eagles are more advanced and stand at the apex of the family Accipitridae, along, perhaps, with such "modernized" and cosmopolitan genera as Circus and Accipiter, with Buteo itself only a shade below them. The genus Buteo thus forms a standard of comparison between the harpy group on the one hand and the booted eagles on the other. Osteologically, Buteo and Aquila are said to be extremely similar. As to the living species comprising the two genera, however, there is not one that calls for the slightest hesitation in generic placement. Genera of Booted Eagles 111 Relevance of Fossils Eventually comparison of fossils will aid in classifying the living booted eagles but as yet the fossil material is so scanty and fragmentary as to be of little help, especially since the living forms are very closely and intricately related to one another. A few fossil forms have been assigned to living genera but, with the exception of some Pleistocene forms such as the Golden Eagles of the California tar pits (Howard 1947), without good comparative osteological material.