Verreaux-Uhu

Bestimmung

Despite the alternative common name of giant eagle-owl, Verreaux's eagle-owl is not the largest owl or eagle-owl in the world. It is, however, a very large and powerful owl species. This species is both the largest owl found in Africa and the world's largest owl to occur in the tropics. Among all the world's owls, it is fourth heaviest living owl, after Blakiston's fish owl (Ketupa blakistoni), the Eurasian eagle-owl (Bubo bubo) and the tawny fish owl (Ketupa flavipes). In addition, it is the fourth longest extant owl (measured from the bill to the tip of the tail), after the great gray (Strix nebulosa), Blakiston's fish and Eurasian eagle-owls. Based on body mass and wing chord length, Verreaux's eagle-owl is about the same size as "medium-sized" races of Eurasian eagle-owl, such as those from Central Asian steppe (B. b. turcomanus) and the Himalayas (B. b. hemachalana), slightly smaller than most northern Eurasian races, considerably smaller than Siberian and Russian eagle-owls, and somewhat larger than the smallest Eurasian eagle-owl subspecies, such as those from the Iberian Peninsula (B. b. hispanus) and the Middle East (B. b. omissus or nikolskii).<p>Verreaux's eagle-owl ranges from 58 to 66 cm (23 to 26 in) in total length. This species has been reported as having an average wingspan of 140 cm (4 ft 7 in), but Mikkola referenced this as the wingspan of a smaller male. The largest known wingspan from a wild female measured nearly 164 cm (5 ft 5 in). While female owls are almost always larger than males, Verreaux's eagle-owl stands out as one of the most sexually dimorphic living owl species, some studies showing the female can average 35% heavier than the male. In comparison, the females of the nominate subspecies of Eurasian eagle-owls and great horned owl (Bubo virginianus) are reported to average approximately 20% and 25% heavier than the males, respectively. The full range of reported body mass in the species ranges from 1,615 to 2,000 g (3.6 to 4.4 lb) in males against a body mass of 2,475–3,150 g (5.5–6.9 lb) in females. In one study, 4 males were found to have averaged 1,704 g (3.8 lb) while 6 females averaged 2,625 g (5.8 lb). Another study found 5 males to have averaged approximately 1,700 g (3.7 lb) while five females averaged 2,300 g (5.1 lb). Unusually large sizes have been claimed in captivity with claims that specimens measuring up to 75 cm (30 in) in length and 200 cm (6 ft 7 in) in wingspan but these are unverified and possibly misreported as these figures match the largest Eurasian eagle-owls. Males heavier than any in the wild have been verified in captivity to weigh up to 2,200 g (4.9 lb). Among standard measurements, the female is reported to measure from 447 to 490 mm (17.6 to 19.3 in), averaging 465 mm (18.3 in), in wing chord, 230 to 273 mm (9.1 to 10.7 in) in the tail, while the same measurements in the male are from 420 to 490 mm (17 to 19 in), averaging 448 mm (17.6 in), and from 220 to 275 mm (8.7 to 10.8 in) in tail length. In both sexes, the tarsus has measured 73 to 86 mm (2.9 to 3.4 in) and the bill (in a small sample) 51 to 54 mm (2.0 to 2.1 in). Based on wing chord size compared to body mass and other linear dimensions, the Verreaux's eagle-owl averages somewhat larger in the size of its wings relative to its body size than most other eagle-owls, excluding the Asian fish owls which are also relatively long-winged.<p>Overall, Verreaux's eagle-owl is a fairly uniform and somewhat pale gray, with light and fine brownish vermiculations on the underside. The back is more solidly light brown with white spots on the shoulder. The oval facial disc is paler, sometimes ranging into a whitish color, than the rest of the front side of the bird with strong black borders bracketing either side. One other feature that immediately distinguishes adult Verreaux's eagle-owls in good light are its pink eyelids. The ecological purpose of their colorful eyelids is not known; however, Brown (1965) opined that they replace the colorful yellow to orange eyes of eagle-owls in breeding and territorial displays, since they were very conspicuous in displaying males. Their eyes are dark-brown in color and like all eagle-owls, they have ear-tufts. The ear-tufts are blunter and smaller relative to those of other African eagle-owls. The ear-tufts of this species are relatively subtle and can be missed in the field, especially if they are held lax. In appearance, they are quite easily distinguished if seen well. They are much bigger and bulkier than most other co-occurring owls. The only eagle-owl species in range that approaches its size is the Shelley's eagle-owl (Ketupa shelleyi), which may (but is not confirmed to) co-exist with the Verreaux's in northern Cameroon and the southern sliver of the Central African Republic most likely in forest edge and mosaics, but that species is a much darker sooty colour overall with broad black bands on the underside. Shelley's eagle-owl also has considerably different habitat preferences, preferring deep, primary forests, and is much more rarely observed in the wild.<p>The next largest owl in sub-Saharan Africa is the Cape eagle-owl (Bubo capensis). The individual home ranges, if not habitats, of the Verreaux's and cape eagle-owls may abut in nearly every part of the latter's distribution. Even in its largest race (Mackinder's eagle-owl, B. c. mackinderi) the cape eagle-owl is around 30% lighter in body mass on average than the Verreaux's eagle-owl, not to mention it being markedly different in almost all outward characteristics. Pel's fishing owl (Scotopelia peli), which occurs in west, central and inland southern Africa and may co-exist with the Verreaux's eagle-owl in much of its range (despite favoring wetland and riparian zones surrounded by wooded areas), can attain similar sizes as the Verreaux's eagle-owl but is dramatically different in color (a rather brighter rufous-cinnamon hue) and lacks ear-tufts. In combination, the characteristics of their pink eyelids, dark eyes, relatively uniform plumage and extremely large size render the Verreaux's eagle-owl as nearly unmistakable.<p>The call of the Verreaux's eagle-owl is the deepest of any extant owl species and one of the deepest bird calls in the world, averaging slightly deeper than the calls of the Blakiston's and brown fish owls (Ketupu zeylonensis). The calls of Eurasian eagle-owls are less deep but are possibly louder and farther carrying. The male's song is an exceptionally deep gwok, gwok, gwonk-gwokwokwok gwokwokwok gwonk. The depth and quality of the song makes confusion by sound more likely with a leopard (Panthera pardus) than any other bird. The song is sometimes considered unmistakable. According to a study in Kenya, the voice is considered the second deepest bird call after the southern ground hornbill (Bucorvus leadbeateri), though that species has a fairly croaking call reminiscent of a large frog and in recordings appears to have a less sonorous call. Apparently, the song can carry up to 5 km (3.1 mi) away on quiet nights. The female's call is similar but higher pitched, as in all owls to some extent because the larger female tends to have a smaller syrinx. Like most Bubo owls, breeding pairs not infrequently call together but they are not as well-synchronized as the pair duets of spotted eagle-owls (Bubo africanus), which are often found in nearby ranges. The alarm calls of both sexes are often a sonorous whok or hook but variable grunting notes and raspy screams also seem to indicate alarm. Both the female and the young engage in high, piercing calls when begging for food at the nest (at which time the male does the food capture). One other vocalization recorded has included a raspy, drawn-out shrooooo-ooo-eh apparently uttered as a distraction display mainly by the male near the nest. While sound is important to some degree for inner-species relations and hunting behaviour to all owl species, the Verreaux's eagle-owl appears to have relatively small and uncomplicated ear openings compared to several smaller types of owl. This indicates that the auditory senses are relatively unimportant in this species compared to vision.

Taxonomie

There are no known subspecies in the Verreaux's eagle-owl, and there is remarkably little variation in their appearance across their considerable distribution. Reportedly, birds in the southern part of their range appear marginally larger on average but these size differences are quite subtle and may be considered as a mild case of Bergmann's rule. While genetic research has been undertaken for this species, its closest living relative in the genus Ketupa is not fully clear. At one time, the Verreaux's eagle-owl was mentioned as an owl with particularly mysterious genetic alliances among living owls. Per Konig &amp; Weick (2008), the species with studied genetic markers found to be most closely related are a dark-eyed species pair of Asian eagle-owls, the spot-bellied (Ketupa nipalensis) and barred eagle-owls (Ketupa sumatrana), but these are not particularly closely related to the Verreaux's.<p>Among species with available genomes to study for DNA characteristics, it has been revealed that the fish owls, in particular the brown fish owl (Ketupa zeylonensis), is the third most closely related species to the Verreaux's. Notably, Konig &amp; Weick did not test the DNA of other African eagle-owls that may bear relation to the Verreaux's eagle-owl based largely on their solid dark brown eyes, namely Fraser's (Ketupa poensis), greyish (Bubo cinerascens) and Shelley's eagle-owl, as opposed to other eagle-owls which have yellow to orange irises. Fraser's and Usambara eagle-owls also have a small amount of bare skin around their eyes but this tends to bluish in color and is not nearly as extensive as the pink seen in Verreaux's. Other large owls native to Africa, the fishing owls, also have uniform dark brownish eyes and are sometimes included with the genus Bubo but how closely related they are to modern eagle-owls is unclear. Pliocene fossil Bubo owls with clear similarities based on osteological characteristics to the modern Verreaux's eagle-owl (most are currently classified as Ketupa cf. lactea) from South Africa and Tanzania, indicate that the Verreaux's eagle-owl descended from slightly smaller ancestors that increased in size as they diversified from related species.

Wissenschaftlicher Name

Ketupa lactea

Autorität

(Temminck, 1820)

Ordnung

Strigiformes

Familie

Strigidae

Gattung

Ketupa

Verbreitung und Lebensraum

Verreaux's eagle-owl is found through most of sub-Saharan Africa, though it is absent from most of the deep rainforests. The species is found at the highest densities in eastern and southern Africa. As this species avoids primary forests, it is found very spottily in west Africa. Their western distribution includes The Gambia, Senegal, Guinea and Sierra Leone. Eastward from those countries to the Central African Republic, the species is distributed in a narrow transitional zone between the Sahara and rainforests. Seemingly isolated populations occur in central Nigeria and central Mali. In south-western Africa, they range up to as far north as the southern parts of the Republic of the Congo and the Democratic Republic of the Congo, through most of Namibia (excluding the coastal regions) and northern South Africa. In east Africa, their distribution is more or less continuous from southern Sudan, Eritrea and inland Somalia down to South Africa as far as the region of the city of Durban.<p>This species inhabits mainly savanna with scattered trees and thorny vegetation. Verreaux's eagle-owls mainly inhabit rather dry regions, some bordering semi-arid areas. In central Mali, for example, near the extreme northwestern limit of the species range, the habitat that hosts these owls averages less than 55 cm (22 in) of rainfall annually. They also range into riverine forest adjacent to savanna and small, semi-open woodland surrounded by open country, though they are less likely to inhabit heavily wooded habitats. South African eagle-owls are not infrequently found around floodplains and marshes, which may provide the primary nesting habitat in some areas. In Uganda, they are largely associated with riparian woodlands. Verreaux's eagle-owl may live at nearly all elevations, from sea level to near the snow-line at around 3,000 m (9,800 ft) in elevation, such as in the Eastern Rift mountains. However, in general, they only sporadically inhabit rocky areas and so are generally very scarce in mountainous regions. The bushveld of southern Africa is near ideal habitat for Verreaux's eagle-owl and the species may be found at near peak numbers here. The species was historically rare to absent from the Kalahari desert, but the introduction by man of invasive trees like conifers, eucalyptus and acacias, irrigation areas and prey species has allowed them to spottily occupy this region.

Zugverhalten

Resident species

Verhalten

Verreaux's eagle-owls are nocturnal birds and roost by day in trees, with large, shaded horizontal branches of tall, old trees being preferred. In Kenya, the most often-used perch trees were Croton megalocarpus and invasive Eucalyptus. Elsewhere, Acacia trees may be used habitually. Despite normally choosing dense foliage to rest in, sometimes they may sit wherever their hunting path ends from the prior night, including relatively exposed perches. They reportedly sleep rather lightly and will awaken very quickly to defend themselves from attack in daylight hours. Family groups consisting of breeding pairs and their offspring frequently roost together and may engage in allopreening during this time. Reportedly some family groups include eagle-owls that had hatched up to three years prior, which if accurate is exceptional for any type of owl species. During extremely hot days, this species may flutter its throat for cooling purposes and has been known to bathe in rain and shallow water during extreme heat in the middle of the afternoon but usually drinks when possible during nighttime. Each breeding pair of Verreaux's eagle-owl defends a territory and these may be extremely large, ranging in size up to 7,000 ha (17,000 acres).

Schutzstatus

Bestandstrend

stable