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Accipitridae
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The Accipitridae is one of the two major families within the order Accipitriformes (the diurnal birds of prey). Many well-known birds, such as hawks, eagles, kites, harriers and Old World vultures are included in this group. The Osprey is usually placed in a separate family (Pandionidae), as is the Secretary bird (Sagittariidae), and the New World vultures are also usually now regarded as a separate family or order. Karyotype data indicated that the accipitrids hitherto analyzed are indeed a distinct monophyletic group, but whether this group should be considered a family of the Falconiformes or one or several order(s) on their own is a matter of taste.
The accipitrids are a family of small to large birds with strongly hooked bills and variable morphology based on diet.
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Encyclopedia
The Accipitridae is one of the two major families within the order Accipitriformes (the diurnal birds of prey). Many well-known birds, such as hawks, eagles, kites, harriers and Old World vultures are included in this group. The Osprey is usually placed in a separate family (Pandionidae), as is the Secretary bird (Sagittariidae), and the New World vultures are also usually now regarded as a separate family or order. Karyotype data indicated that the accipitrids hitherto analyzed are indeed a distinct monophyletic group, but whether this group should be considered a family of the Falconiformes or one or several order(s) on their own is a matter of taste.
The accipitrids are a family of small to large birds with strongly hooked bills and variable morphology based on diet. They feed on a range of prey items from insects to medium-sized mammals, with a number feeding on carrion and a few feeding on fruit. The Accipitridae have a cosmopolitan distribution, being found on all the world's continents (except Antarctica) and a number of oceanic island groups. Some species are migratory.
Systematics
The accipitrids have been variously divided into some 5-10 subfamilies. Most of them share a very similar morphology, but many of these groups contain taxa which are more aberrant. These are placed in their respective position more for lack of better evidence than anything else. It is thus not very surprising that the phylogenetic layout of the accipitrids has always been a matter of dispute.
As mentioned above, the accipitrids are recognizable by a peculiar rearrangement of their chromosomes. Apart from this, morphology and mtDNA cytochrome b sequence data gives a confusing picture of these birds' interrelationships. What can be said is that the hawks, kites, eagles and Old World vultures as presently assigned in all likelihood do not form monophyletic groups:
According to the molecular data, the Buteoninae are most likely poly- or paraphyletic, with the true eagles, the sea eagles, and the buteonine hawks apparently representing distinct lineages. These appear to form a group with the Milvinae, Accipitrinae and Circinae but the exact relationships between the lineages are not at all robustly resolvable with the present data. The Perninae and possibly the Elaninae are older lineages, as are the Old World vultures. The latter are fairly likely also poly- or paraphyletic, with some aberrant species like the Bearded and Egyptian Vultures standing apart from the naked-necked "(not so)true" vultures.
Morphology
The Accipitridae are a diverse family with a great deal of variation in size and shape. They range in size from the tiny Pearl Kite, which is 25 cm in length and weights no more than 100 g, to the Philippine Eagle, which is 1 m in length and which may weight up to 7 kg. The Old World vultures may weigh as a much as 12 kg. Until the 14th century even these were surpassed by the extinct Haast's Eagle of New Zealand, which is estimated to have weighted 14 kg. Most Accipitridae exhibit sexual dimorphism in size, although unusually for birds it is the females that are larger than the males. This sexual difference in size is more pronounced in active species that hunt birds, and is less pronounced in rodent hunters, and almost non-existent in carrion or snail eaters.
The beaks of accipitrids are strong, hooked (sometimes very hooked, as in the Hook-billed Kite or Snail Kite). In some species there is a notch or 'tooth' in the upper mandible. In all accipitrids the base of the upper mandible is covered by a fleshy membrane called the cere which is usually yellow in colour. The tarsi of different species vary by diet, those of bird hunting species like sparrowhawks are long and thin, while species that hunt large mammals have much thicker, stronger ones, and snake-eagle have thick scales to protect from bites.
The plumage the Accipitridae can be striking but rarely utilises bright colours; most birds use combinations of grey, buff and brown. Overall they tend to be paler below, this helps them seem more inconspicuous when seen from below. There is seldom sexual dimorphism in plumage, when it occurs the males are brighter or the females resemble juveniles. In many species juveniles have a distinctly different plumage. Some accipitrids mimic the plumage patterns of other hawks and eagles. They may attempt to resemble a less dangerous species in order to fool prey, or instead resemble a more dangerous species in order to reduce mobbing by other birds. Several species of accipitrid have crests used in signalling, and even species without crests can raise the feathers of the crown when alarmed or excited. In contrast most of the Old World vultures possess bare heads without feathers; this is thought to prevent soiling on the feathers and aid in thermoregulation.
The senses of the Accipitridae are adapted to hunting (or scavenging), and in particular their vision is legendary. The sight of some hawks and eagles is up to 8 times better than that of humans. Large eyes with two fovea provide binocular vision and a "hawk eye" for movement and distance judging. In addition have the largest pectens of any birds. The eyes are tube shaped and cannot move much in their sockets. In addition to excellent vision many species have excellent hearing, but unlike owls sight is generally the principal sense used for hunting. Hearing may be used to locate prey hidden in vegetation, but sight is still used to catch the prey. Like most birds the Accipitridae generally have a poor sense of smell; even the Old World vultures make no use of the sense, in contrast to the New World vultures in the family Cathartidae.
Diet and feeding
Accipitrids are predominately predators and most species actively hunt for their prey. A few species may opportunistically feed on fruit and in one species, the Palm-nut Vulture, it forms the major part of the diet. However other animals form the bulk of the diet of most species. Insects are taken exclusively by around 12 species, in great numbers by 44 additional species, and opportunistically by many others. The Snail Kite and Hook-billed Kites are specialists in consuming snails. Bazas and forest hawks in the genus Accipiter may take reptiles from trees whilst other species hunt them on the ground. Snakes in particular are targeted by the snake-eagles (Circaetus) and serpent-eagles (Spilornis and Dryotriorchis).
Genera
- Subfamily Elaninae - elanid kites (8 species)
- Genus Elanus
- Genus Chelictinia
- Genus Gampsonyx
- Genus Elanoides
- Subfamily Perninae - honey-buzzards (c.14 species)
- Subfamily Aegypiinae - Old World vultures
- Genus Sarcogyps
- Genus Aegypius
- Genus Torgos
- Genus Trigonoceps
- Genus Gyps
- Genus Necrosyrtes
- Subfamily Gypaetinae
- Genus Neophron
- Genus Gypohierax
- Genus Gypaetus
- Genus Eutriorchis
- Subfamily Buteoninae - buteonine hawks, true eagles and sea-eagles (c.100 living species, probably poly- or paraphyletic)
- Genus Geranoaetus
- Genus Buteo (probably paraphyletic, might include Leucopternis in part and Parabuteo)
- Genus Parabuteo
- Genus Buteogallus (probably paraphyletic, might include Leucopternis in part)
- Genus Busarellus
- Genus Leucopternis (probably polyphyletic)
- Genus Kaupifalco
- Genus Butastur
- Genus Harpyhaliaetus
- Genus Geranospiza
- Subfamily Aquilinae
- Subfamily Circinae - harriers (some 16 living species)
- Subfamily Polyboroidinae - harrier hawks
- Subfamily Milvinae - milvine kites (some 14 species)
- Genus Harpagus
- Genus Ictinia
- Genus Rostrhamus
- Genus Helicolestes - formerly included in Rostrhamus
- Genus Haliastur
- Genus Milvus
- Genus Lophoictinia
- Genus Hamirostra
- Subfamily Accipitrinae - goshawks, sparrowhawks, and relatives (c.55 living species)
- Subfamily Circaetinae - snake-eagles (about one dozen species)
- Subfamily Haliaeetinae
- Subfamily Harpiinae
- Genus Morphnus
- Genus Harpia
- Genus Harpyopsis
- Genus Harpagornis
- Subfamily Melieraxinae
Fossil record
Like with most other birds of prey, the fossil record of this group is fairly decent from the latter Eocene onwards (c.35 mya), with modern genera being well documented since the Early Oligocene, or around 30 mya.
- Milvoides (Late Eocene of England)
- Aquilavus (Late Eocene/Early Oligocene - Early Miocene of France)
- Palaeocircus (Late Eocene/Early Oligocene of France)
- Palaeastur (Agate Fossil Beds Early Miocene of Sioux County, USA)
- Pengana (Early Miocene of Riversleigh, Australia)
- Promilio (Agate Fossil Beds Early Miocene of Sioux County, USA)
- Proictinia (Early - Late Miocene/Early Pliocene of C and SE USA)
- Neophrontops (Early/middle Miocene - Late Pleistocene) - formerly in Neophron
- Mioaegypius (Xiacaowan middle Miocene of Sihong, China)
- Apatosagittarius (Late Miocene of Nebraska, USA)
- Palaeoborus (Miocene)
- Qiluornis (Miocene of Shandong, China)
- Thegornis (Miocene of Argentina)
- Garganoaetus (Early Pliocene of Gargano Peninsula, Italy)
- Amplibuteo (Late Pliocene of Peru - Late Pleistocene of S North America and Cuba) - may belong to extant genus Harpyhaliaetus
- Neogyps
- Palaeohierax - includes "Aquila" gervaisii
- Wetmoregyps - formerly Morphnus daggetti
Accipitrids are known since Early Eocene times, or about from 50 mya onwards, in fact, but these early remains are too fragmentary and/or basal to properly assign a place in the phylogeny. Likewise, as remarked above, molecular methods are of limited value in determining evolutionary relationships of and within the accipitrids. What can be determined is that in all probability, the group originated on either side of the Atlantic, which during that time was only 60-80% its present width. On the other hand, as evidenced by fossils like Pengana, some 25 mya, accipitrids in all likelihood rapidly acquired a global distribution - initially probably even extending to Antarctica.
- Accipitridae gen. et sp. indet. (Huerfano Early Eocene of Huerfano County, USA)
- Accipitridae gen. et sp. indet. (Borgloon Early Oligocene of Hoogbutsel, Belgium)
- Accipitridae gen. et sp. indet. (Bathans Early/Middle Miocene of Otago, New Zealand)
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"Aquila" danana (Snake Creek Late Miocene/Early Pliocene of Loup Fork, USA) - formerly also Geranoaetus or Buteo Accipitridae gen. et sp. indet. (Early/Middle Pliocene of Kern County, USA) - Parabuteo? Accipitridae gen. et sp. indet. (Late Pliocene/Early Pleistocene of Ibiza, Mediterranean) - Buteo? Accipitridae gen. et sp. indet. (Egypt)
Specimen AMNH FR 2941, a left coracoid from the Late Eocene Irdin Manha Formation of Chimney Butte (Inner Mongolia) was initially assessed as a basal mid-sized "buteonine"; it is today considered to be more likely to belong in the Gruiformes genus Eogrus.
Footnotes
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