Tawny fish owl

Tawny fish owl
Scientific classification
Kingdom: Animalia
Phylum: Chordata
Class: Aves
Order: Strigiformes
Family: Strigidae
Genus: Bubo (but see text)
Species: B. flavipes
Binomial name
Bubo flavipes
(Hodgson, 1836)
Synonyms

Cultrunguis flavipes Hodgson, 1836[2]

The tawny fish owl (Bubo flavipes) is a species of owl. It used to be placed in Ketupa with the other fish owls, but that group is tentatively included with the eagle-owls in Bubo, until the affiliations of the fish owls and fishing owls can be resolved more precisely. It is clear from several shared characteristics that the more typical Bubo and fish owls are indeed related, including the structure of the talons, prominent ear tufts and plumage characteristics, unlike the superficially dissimilar fishing owls of Africa.[3]

Distribution and habitat

This typical owl is found in subtropical to temperate forests in Bangladesh, Bhutan, China, India, Laos, Myanmar, Nepal, Taiwan, and Vietnam. They inhabit the Himalayan foothills from Kashmir and Garhwal east to the mountains of Laos and Vietnam and in southern China up to Chekiang and Anhwei. They require forest tracts and duars bearing mountain streams. In areas such as Darjeeling and Nepal, they commonly live at elevations of 1,500 to 2,450 m (4,920 to 8,040 ft).[3] It more less replaces the buffy fish owl (Bubo ketupu) in range and at one time they were considered to be of the same species. The tawny fish owl may overlap in range with brown fish owls (B. zeylonensis) in Laos and Vietnam, the brown usually but not always inhabiting lowland areas and preferring slow or stagnant waters unlike the tawny which prefers faster flowing waters. Unlike the brown and buffy fish owls, they are found only in remote wilderness with little to no disturbance. Those other two fish owls (but not the Blakiston's species) can adapt to living and hunting near human habitations or human-altered areas, such as canals, paddy fields, reservoirs, ornamental ponds and villages, which provides them a boon of ornamental and commercial fishes.[3]

Description

Like other fish owls, the tawny fish owl has large ear tufts but they usually hang to the sides of the head and are distinctly messy and toussled looking. They have yellow eyes. Tawny fish owls have been described as the most "attractive" of the fish owls.[4] They tend to be an orangey-rufous color on the crown and upperparts, which are overlaid with broad, blackish markings on the central part of the feathers and spots of the same color as the reddish-brown feather edges. The scapulars are a dingy yellow color, forming a contrasting band which runs across the owl's shoulders. The flight and tail feathers are strongly barred dark brown and buffish. The facial disc is poorly defined but a sizeable off-white area on the eyebrows and forehead stands out. While buffy and brown fish owls are featherless on their legs and the Blakiston's fish owl (B. blakistoni) has totally feathered legs (the latter more like most Bubo), the tawny fish owl has feathering over two-thirds of the tarsi. The legs below feathering are greenish-yellow with greyish-horn coloured talons. Beside the variability of the feathering of the legs, the buffy fish owl is most similar in plumage but is smaller and buff hued rather than orange-rufous hued. The brown fish owl is a much more solid brown color with distinct vermiculations below and no yellowish band across the back.[5][6]

Size and physiology

Compared to eagle owls of similar length, fish owls tend to be even shorter in tail length and even heavier in build, have relatively larger wings (the tawny and Blakiston's being particularly chunky in shape), have considerably longer legs, and have a rough texture to the bottom of their toes. At least the latter two features are clear adaptations to aid these owls in capturing fish. Diurnal raptors who feed largely on fish have similar, if not identical, rough texture under their toes, which helps these birds grasp slippery fish. Unlike diurnal raptors who capture fish such as the osprey (Pandion haliaetus) as compared to most terrestrial raptors, the fish owls have large, powerful, and curved talons and a longitudinal sharp keel sitting under the middle claw with all having sharp cutting edges that are very much like those of eagle owls. Also, unlike fish-eating diurnal raptors will not submerge any part of their body while hunting, preferring only to put their feet into the water, although fish owls will hunt on foot, wading into the shallows. Unlike most owls, the feathers of fish owls are not soft to the touch[3] and they lack the comb and hair-like fringes to the primaries, which allow other owls to fly silently in order to ambush their prey. Due to the lack of these feather-specializations, fish owl wing beats make sounds. The lack of a deep facial disc in fish owls is another indication of the unimportance of sound relative to vision in these owls, as facial disc depth (as well as inner ear size) are directly related to how important sound is to an owl's hunting behavior. Similar adaptations, such as unwillingness to submerge beyond their legs and lack of sound-muffling feathers are also seen in the African fishing owls, which do not seem to be directly related. Tawny fish owls are around the same size as the brown fish owl in terms of linear dimensions. The tawny is 48 to 61 cm (19 to 24 in) long from bill to tail. However, studies have revealed the tawny is surprisingly rather heavier on average than the brown fish owl and, less surprising, is considerably heavier than an eagle owl of comparable length. Six adult tawny fish owls were found to weigh an average of 2,415 g (5.324 lb), with a reported range of 2,050 to 2,650 g (4.52 to 5.84 lb), and are thus one of the heaviest living owl species. Only the Blakiston's fish owl and a majority of races of Eurasian eagle owl (B. bubo) are heavier on average. The maximum (not average) weight of Verreaux's eagle owls (B. lacteus) and snowy owls (B. scandiacus) are also higher but those species have far larger sample sizes of body mass.[7][8] In terms of standard measurements, the wing chord is 410 to 477 mm (16.1 to 18.8 in), the tail is 215 to 227 mm (8.5 to 8.9 in), the tarsus is 60 to 67 mm (2.4 to 2.6 in) and the bill is 48 to 52 mm (1.9 to 2.0 in). Compared to the brown fish owl, the tawny averages of similar tail length, is larger in size by average wing length and bill size and slightly smaller in tarsal length.[6][9][10]

Voice

Their territorial call has been described as a deep whoo-hoo. A cat-like meow has also been described for the tawny fish owl, with the brown fish owl also making a similar vocalization.

Behavior

Tawny fish owl in Jim Corbett National Park, Uttarakhand, India.

The tawny fish owl is at least partially diurnal in activity, with daytime activity mainly occurring in the late afternoon and they may be seen actively hunting before nightfall especially on cloudy days. However, before the afternoon they tend to be sluggish during the day. If disturbed or threatened, these owls tend to sit tight and not take flight. Like most owls, they usually choose inconspicuous perches during the day to avoid detection. The tawny fish owl has been described as the "most powerful and savage" of the three smaller fish owl species. However, in a majority of dietary habits, they are broadly similar to other fish owls and they do not tackle large prey with equal regularity or aplomb as the rather larger Blakiston's fish owl.[4] They usually hunt by swooping down to the water capture fish from the surface and are reportedly surprisingly active in their hunting style and are not dissimilar in the hunting methods to those used by diurnal fish-hunting raptors such as fish eagles, sea eagles and ospreys. Tawny fish owl mainly eat fish, crabs, shrimps, crayfish and frogs. The primary food of tawny fish owls in one study from Taiwan was freshwater crabs (apparently of the Candidiopotamon genus) which made up 62.8% of the diet, followed by the Asiatic toad (Bufo gargarizans), at least three other frog species, then fish and Eriocheir mitten crabs. The toads were taken considerably more regularly than other frog species, although far less abundant in number in the stream wetlands there, due to their larger sizes.[11] More terrestrial prey is by no means avoided though and the species may also hunt toads, lizards, snakes, and small mammals such as moles, and particularly rodents, with one of the few mammalian prey recorded semi-regularly being bamboo rats (Rhizomys). A small Malayan porcupine (Hystrix brachyura) has also been reported amongst their prey. It also prey on birds including Mandarin ducks (Aix galericulata) in Taiwan and has overtaken large ground birds such junglefowl (Gallus ssp.), pheasants and eared pheasants, the latter sometimes weighing more than 2 kg (4.4 lb). Tawny fish owls tend to be sparsely distributed and frequently occupies a stretch of riparian zone of 5.5–7.7 km (3.4–4.8 mi) in length.[3][6]

Tawny fish owls are highly solitary and territorial as are a majority of owls. The breeding season is November to February in India and December to February in Assam. Nest locations found have included large holes in river banks, caves in cliffs and the fork or crotch of a large tree. As in all owls, tawny fish owls do not build a nest so merely lay their eggs on the bare ground of whatever surface they use. They also not infrequently nest in abandoned nests built by Pallas's fish eagles (Haliaeetus leucoryphus). Usually two eggs are laid but sometimes only one is. The eggs can range in size from 56 to 58.8 mm (2.20 to 2.31 in) x 45.5 to 48.3 mm (1.79 to 1.90 in), with an average of 57.1 mm × 46.9 mm (2.25 in × 1.85 in), and are similar in size to those of the brown fish owl. Greater details of the reproductive biology are not currently known although are presumed to basically be similar to those of other fish owls.[3][6]

Notes

  1. ↑ BirdLife International (2012). "Ketupa flavipes". IUCN Red List of Threatened Species. Version 2012.1. International Union for Conservation of Nature. Retrieved 16 July 2012.
  2. ↑ Hodgson, BH (1836). "On a new Piscatory Genus of the Strigine Family". Journal of the Asiatic Society of Bengal. 5: 363–365.
  3. 1 2 3 4 5 6 Voous, K.H. 1988. Owls of the Northern Hemisphere. The MIT Press, 0262220350.
  4. 1 2 Hume, R. (1991). Owls of the world. Running Press, Philadelphia, PA. 1991.
  5. ↑ Owls of the World: A Photographic Guide by Mikkola, H. Firefly Books (2012), ISBN 9781770851368
  6. 1 2 3 4 König, Claus; Weick, Friedhelm (2008). Owls of the World (2nd ed.). London: Christopher Helm. ISBN 9781408108840.
  7. ↑ CRC Handbook of Avian Body Masses, 2nd Edition by John B. Dunning Jr. (Editor). CRC Press (2008), ISBN 978-1-4200-6444-5.
  8. ↑ TANA, P. G., VAZQUEZ, A., & CHAVEZ, C. (1997). NOTES ON A NEST OF THE TAWNY FISH-OWL (KETUPA FLAVIPES) AT SAKATANG STREAM, TAIWAN. Wilson Bulletin, 66, 135-136.
  9. ↑ Weick, F. (2007). Owls (Strigiformes): annotated and illustrated checklist. Springer.
  10. ↑ Robson, C., & Allen, R. (2005). New Holland field guide to the birds of South-East Asia. New Holland Publishers.
  11. ↑ Wu, H. J., Sun, Y. H., Wang, Y., & Tseng, Y. S. (2006). Food habits of Tawny Fish-Owls in Sakatang Stream, Taiwan. Journal of Raptor Research, 40(2), 111-119.

References

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