The Evolution and Classification of Old World Vultures

As the location of Old World vultures in the Falconiformes is less disputed, and the classification controversies are less intense than those for the New World vultures, this section dealing with the Old vultures is shorter than the previous section covering the New World vultures. Here we look at the origins and relations of Old World vultures within the larger Order Falconiformes and Family Accipitridae. Fundamentally, are Old World vultures one group of birds with one ancestor, or several groups with different ancestors, and which raptors are their closest relatives—eagles, sea eagles, buzzards or kites?

As was mentioned earlier, Old World vultures contrast with the New World vultures, as they have stronger, grasping, feet and talons, a voice box and rounded nasal openings (Feduccia 1999). Their ancestors are held to be eagle-like, in fact two species, the Bearded vulture and the Palm-nut vulture, are commonly seen as intermediate between vultures and eagles (Feduccia 1999). The White-head vulture is also seen as having some eaglelike attributes. The fossil record of Old World vultures occurs in both the Old and the New Worlds from about the Late Oligocene, with increased numbers up to the Pleistocene (Zhang et al. 2012a). Old World vultures were widespread in both the Old World and North America, during the Neogene (Wink 1995), the Early Miocene of North America (Palaeoborus) and the Late Oligocene of Europe (Palaeohierax) (Miller and Compton 1939; Brodkorb 1964).

Zhang et al. (2012b: 1) argue that 'among the great surprises of avian paleontology was that compared with the Old World, the New World has an unexpectedly diverse and rich fossil component of Old World Vultures.' According to Feduccia (1999) in 1916, Miller described a fossil specimen of Neophrontops americanus and Neogyps errans, from the Pleistocene Rancho La Brea tar pits of southern California. The former was closely related to the modern Egyptian vulture Neophron percnopterus (Savigny, 1809) (Miller and Demay 1942). The status of these Old World vultures in the New World is disputed (Rich 1980); however, the balance of opinion seems to favor Old World species in the New World, especially in the case of Neophrontops, which is 'markedly like that of the Recent Old World vulture Neophron' (Howard 1966b: 3), especially as the differences between these two species 'are of less note than those which exist between Neophron and its contemporaries among the vultures today' (Howard 1932, 70).

Concerning the current Old World vulture species, two subfamilies are usually recognized; (1) the Gypaetinae, including Genus Gypaetus (Lammergeier or Bearded Vulture Gypaetus barbatus); the Genus Gypohierax (Palm-nut Vulture Gypohierax angolensis); and Genus Neophron (Egyptian Vulture Neophron percnopterus); and (2) the much larger Aegyptiinae, containing the Genus Aegypius (Cinereous vulture, Aegypius monachus); Genus Gyps (Griffon Vulture Gyps fulvus, White-rumped vulture Gyps bengalensis, Ruppell's vulture, Gyps rueppelli, Indian vulture Gyps indicus, Slender-billed vulture, Gyps tenuirostris, Himalayan vulture Gyps himalayensis, White-backed Vulture Gyps africanus, Cape Vulture, Gyps coprotheres; the Genus Necrosyrtes (the Hooded Vulture, Necrosyrtes monachus) Genus Sarcogyps (Red-headed vulture Sarcogyps calvus); Genus Torgos (Lappet-faced vulture Torgos tracheliotus); and the Genus Trigonoceps White-headed Vulture Trigonoceps occipitalis.

As noted above, in the past, vultures were once thought to have evolved 'only once among extant diurnal birds of prey' (Sielbold and Helbig 1995: 163). The Old World vultures have generally always been considered as within the raptors. Therefore disputes on their classification usually concern the closeness of the relation between particular vultures and other raptors (especially some eagles, kites and buzzards), and the possibility of some species (e.g., the feather-headed Bearded vulture and Palm-nut vulture) being not fully vultures, but between vultures and other raptors.

Old World vultures (Aegypiinae and Gypaetinae) have been proposed to be monophyletic (Brown and Amadon 1968; Thiollay 1994) or polyphyletic with Gyophierax, Neophron, and Gypaetus forming one or more groups separate from the others (Jollie 1977b; Mundy et al. 1992; Seibold and Helbig 1995). In this sense as noted above, a monophyletic group is a taxon (group of organisms) forming a clade, i.e., consisting of an ancestral species and all its descendants. A polyphyletic group has convergent features not inherited from a common ancestor, but from several ancestral species. Griffiths et al. (2007) point out that the subfamilies of the Accipitridae have usually been based on differences in behavior and feeding ecology, and physical similarities and differences. As such distinctions may not have a genetic/ biochemical basis, the subfamily and Genera monophyly is possibly subjective and inaccurate. They argue that phylogeny for Accipitridae based on morphology is difficult to assess (e.g., Brown and Amadon 1968; Jollie 1976, 1977a,b; Thiollay 1994; Seibold and Helbig 1995; Griffiths et al. (2007). Recent phylogenetic analyses have used osteology, DNA sequences from a single mitochondrial gene and mitochondrial plus nuclear DNA sequences (Wink 1995).

An incisive study was that of Lerner and Mindell (2005) who presented a 'full or nearly complete taxonomic representation of five accipitrid subgroups (sea and fish eagles, harpy eagles, booted eagles, snake eagles, and Old World vultures'. Both mitochondrial and nuclear sequences were used for representative species of 51 out of 65 genera (78%) and almost half of the known Accipitridae species (n = 111). The three species in Gypaetinae were highly divergent from the remaining 11 species in Aegypinnae (see also Mundy et al. 1992). Reference was also made to the study by Seibold and Helbig (1995), who used cyt-b sequence from 11 Old World vulture species and found evidence of polyphyly for the Old World vultures. This relates to polarized opinions on Old World vultures, that they are either monophyletic (Brown and Amadon 1968; Thiollay 1994) or polyphyletic with Gyophierax, Neophron, and Gypaetus forming one or more groups separate from the others (Jollie 1977b; Mundy et al. 1992; Seibold and Helbig 1995).

Key issues concern not only the monophyletic or polyphyletic status, but also possible 'sister groups' within the Old World vultures, including other raptors that may be related to vultures (Lerner and Mindell 2005). The Palm-nut vulture may be a transition between vultures to sea eagles (Brown and Amadon 1968). The Gypaetinae (including Gypohierax, Gypaetus, Neophron and the serpent eagle Eutriorchis) would be of a different evolutionary origin, diverging earlier, possibly related to the Perninae hawks (Wink 1995). The Aegypiinae are a monophyletic group (possibly close to the Circaetinae snake eagles and Aquiline eagles (Wink 1995). The 'phylogenetic position of Necrosyrtes (the Hooded vulture) within the Aegypiinae remains uncertain' (Lerner and Mindell 2005). It is more closely related to (but divergent from) the Gyps than the other Aegypinae. Although the Genera Aegyptiinae, Torgos, Aegypius, Sarcogyps and Trigonoceps are classified separately, they may even be within the same genus (ibid.).

These findings were complemented by those of Wink (1995), which went further, and for the part concerning Old World vultures, sought to identify phylogenetic relationships within Old World vultures and between these and other raptors using molecular data. This study found that each of the Gypaetinae species (Gypohierax angolensis, Eutriorchis astur, Neophron percnopterus, and Gypaetus barbatus) was highly divergent from each other genetically, but more closely related to each other than to the other Accipitridae species. A strange result was the finding that Eutriorchis astur, the Madagascan Serpent Eagle is a member of the Gypaetinae. Aegypius monachus and Torgos tracheliotus clustered 'as sister species' (ibid. 875). Necrosyrtes was uncertain in relation to the Gyps or Aegypius-clades, despite morphological similarities with Neophron (del Hoya et al. 1994). The Gyps vultures were closely related, possibly from a common ancestor about 500000 years BP. Gyps africanus and G. bengalensis were a better fit in Gyps than in the separate genus Pseudogyps. and were found to be separate rather than same species (see also Dowsett and Dowsett-Lem aire 1980). Neophron and Gypaetus (the Egyptian and Bearded Vultures) were related, as had been found in the past using karyological (DeBoer and Sinoo 1984) morphological, anatomical (Jollie 1976, 1997a,b) and embryological data (Thaler et al. 1986). The Gypaetus/Neophron group was not close to other Old World Vultures.

There was also a close relation between Short-toed Snake Eagles (Circaetus gallicus Gmelin, 1788), and the Gyps/Aegypius clade was confirmed as it was earlier by morphological studies (Jollie 1976, 1977a,b; Mundy et al. 1992). The Honey buzzard (Pernis apivorus Linnaeus, 1758) was found to be near to Neophron/Gypaetus (which was previously at the base of the accipitrid tree) (Wink 1995).