Blakiston's fish owl (Ketupa blakistoni), the largest living species of owl, is a fish owl, a sub-group of eagle-owls that specialize in hunting in riparian areas. It is native to China, Japan, and the Russian Far East. This species is a part of the family known as typical owls (Strigidae), which contains most species of owl. Blakiston's fish owl and three other piscivorous owls are placed with some eagle-owls in the genus Ketupa. Its habitat is riparian forest with large, old trees for nest sites near lakes, rivers, springs, and shoals that do not freeze in winter. Henry Seebohm named this bird after the English naturalist Thomas Blakiston, who collected the original specimen in Hakodate on Hokkaidō, Japan in 1883.
Taxonomy Blakiston's fish owl was formally described in 1884 by the English amateur ornithologist Henry Seebohm from a specimen collected near Hakodate on the island of Hokkaido in Japan. He placed the owl in the genus Bubo and coined the binomial name Bubo blakistoni. The specific epithet was chosen to honour the naturalist and explorer Thomas Blakiston who had supplied Seebohm with a specimen.
Blakiston's fish owl is more closely related to the Eurasian eagle-owl than to the subgenus Ketupa of fish owls it was formerly believed to be closer to. This was shown by osteological and DNA-based tests in 2003 by ornithologists/taxonomists Michael Wink and Claus König, author of Owls of the World. However, the other fish owls are not believed by some authors to be divergent enough to support a separate genus either and so some authorities list fish owls as also included in the genus Bubo.
Given that it shares genetic material and osteological characteristics with the Eurasian eagle-owl, yet also seems to share some characteristics with the other three fish owls (especially the brown fish owl), the place of the Blakiston's fish owls in this evolutionary chain is ambiguous. Some authors have wondered whether the Blakiston's represents an intermediate step between traditional eagle-owls and the other fish owls, despite the current gap in distribution between Blakiston's and other fish owls. Whether other Asian eagle-owls with sideways slanting ear-tufts, namely the spot-bellied (B. nipalensis), the barred (B. sumatranus), and especially the somewhat superficially fish owl–like dusky eagle-owl (B. coromandus) are closely related to the fish owls and/or the Blakiston's is also unclear.
Although a few authors also include them in Bubo, the fishing owls of Africa (generally classified in the genus Scotopelia) seem to be fairly dissimilar. Based on external characteristics, osteology and preliminary genetic materials, the fishing owls are likely to have some convergently evolved characteristics to the fish owls and have no close relationship. Ultimately, given the lack of specific genetic studies for them, it is not clear how closely fishing owls are related to typical eagle-owls (to which fish owls are related).
Recent genetic testing in Russia indicates a fairly large divergence from the genomes of typical Bubo and suggests that they are indeed more closely aligned with other fish owls.Spiridonova, L.N. & Surmach, S.G. (2018). Whole mitochondrial genome of Blakiston's fish owl Bubo (Ketupa) blakistoni suggests its redescription in the genus Ketupa. Russian Journal of Genetics, 54 (3): 369–373.
Description
Blakiston's fish owl is the largest living species of owl. A pair field study of the species showed males weighing from , while the female, at up to , is about 25% larger.National GeographicTaczanowski, W. (1891). Faune ornithologique de la Sibérie orientale, tt. 1–2, Eggers, St. Around February, the average weight of Russian fish owls was in seven males and in five females, typically when their body mass at its lowest throughout the year.Slaght, J. C., Horne, J. S., Surmach, S. G., & Gutiérrez, R. J. (2013). Home range and resource selection by animals constrained by linear habitat features: an example of Blakiston's fish owl. Journal of Applied Ecology, 50(6), 1350–1357. Blakiston's fish owl measures in total length, and thus measures slightly less at average and maximum length than the great grey owl (Strix nebulosa), a species which has a significantly lower body mass. The Eurasian eagle-owl (B. bubo) is sometimes considered the largest overall living owl species. The three largest species of eagle-owl, all found in Siberia and the Russian Far East, are close in size to the Blakiston's fish owl. According to Heimo Mikkola, the very largest specimen of eagle-owl was longer in bill-to-tail length than the longest Blakiston's fish owl, while the top weight of the two species is exactly the same. The longest great gray owl was longer than the biggest Blakiston's fish owl but would be about 2.5 times lighter than the weight of the largest female Blakiston's. However, the average measurements of Blakiston's fish owl surpass the average measurements of the Eurasian eagle-owl in at least two of the major categories: weight and wingspan, making Blakiston's the overall largest species of owl. Even the large Siberian races of eagle-owl are slightly smaller on average than the Blakiston's, at least in terms of body mass and wing size. In the largest known body mass cited for a Eurasian eagle-owl population, the weights cited, for instance, were a mean for the males and a mean of for the females.Mikkola, H. (1983). Owls of Europe. T. & AD Poyser.Mikkola, H. (2017). The Eurasian Eagle Owl is the largest living owl species! Tyto, 12(1): 15–18. The maximum wingspan of the Blakiston's fish owl is also greater than any known eagle-owl. The wingspan range known for Blakiston's fish owls is . It is possible the largest specimens can attain a wingspan of approximately .Burton, P. (1973). Owls of the World: Their evolution, structure and ecology. New York: Dutton & Co. The Blakiston's is noticeably larger than the other three extant species of fish owl.
In terms of structure, the Blakiston's fish owl is more similar to eagle-owls than it is to other fish owls but it shares a few characteristics with both types of owl. Like all fish owls, its bill is relatively long, the body relatively husky and wings are relatively long compared to eagle-owls. It also shares with other fish owls a comparatively long tarsi, although relative to their size the three smaller fish owl have a proportionately longer tarsus. Other than these few characteristics, a Blakiston's fish owl skull and skeleton is practically the same as that of a Eurasian eagle-owl. The talons of the Blakiston's fish owl are similar in shape and size to those of the Eurasian eagle-owls. It has been stated that the combination of wavy cross patterns on the underside of the Blakiston's plumage and its huge talons make it look strikingly like an outsized great horned owl (B. virginianus) from below. Two external characteristics that Blakiston's share with eagle-owls, but not with the other fish owls, is that its tarsi are totally feathered and that its wing beats are silent, although apparently the Blakiston's has relatively fewer sound-blocking combs on its wing primaries than the a comparable eagle-owl would. Among standard measurements, which at average and maximum are greater than any other living owl other than tail length, the wing chord measures , the tail measures , the tarsus is and the culmen is around .
Superficially, this owl somewhat resembles the Eurasian eagle-owl but is more monochromatically brown to tan in colour. Like other fish owls but unlike most eagle-owls, the Blakiston's fish owl has relatively broad and ragged ear tufts which hang slightly to the side and that do not appear upright. The upperparts are buff-brown and heavily streaked with darker brown coloration. The underparts are a paler buff brown and less heavily streaked. The throat is white. The iris is yellow (whereas the Eurasian eagle-owl typically has an orange iris). The Eurasian eagle-owl and Blakiston's fish owl both occur in the Russian Far East and are potentially could compete for resources, although no scientifically observed interactions of any kind have been reported between these two largest owl species. It is likely, given the sizeable gap between the dietary preferences of the species (mainly aquatic animals in the Blakiston's, and mainly upland, terrestrial species in the eagle-owl), that competition for food is not normally a serious problem. Identification of the Blakiston's from other fish owls is not an issue as there is a gap of distribution of approximately between the ranges of the Blakiston's and the tawny fish owl (B. flavipes) and about separates the range of the Blakiston's and the brown fish owl (Ketupa zeylonensis). Improbably, early naturalist thought that the Blakiston's and brown fish owls belonged to the same species. The streaking on the underside of the brown and Blakiston's are similar and their songs sound more similar to each other than they do with the two songs of the other two species of fish owl, being deeper voiced with a dissimilar vocal pattern to the latter fish owl.
Vocalizations differ among the recognized subspecies. In the nominate subspecies from Japan, the male calls twice and the female responds with one note, whereas the mainland subspecies has a somewhat more elaborate, four-note duet: HOO-hoo, HOBO-hoooo (here, the male call is in capital letters (HOO) and the female call in lower case (hoo)). The transliterations of the calls of owls from Russia, representative of the owl's vocal variations, are SHOO-boo and FOO-foo-foo. The territorial song or call in Russia in particular has been described as somewhat like a short, deep eagle-owl's call. Despite its slightly larger size, the Blakiston's fish owls voice is not as sonorous or as far-carrying as is the Eurasian eagle-owl's voice is. The fish owl's voice is rather deeper, however. As in most owls, vocal activity tends to peak directly before nesting activity begins, so peaks around February in this species. This duet of pairs of Blakiston's fish owl in the period leading up the breeding season is so synchronized that those unfamiliar with the call often think it is only one bird calling. When an individual bird calls, it may sound like hoo-hooo. Juveniles have a characteristic shriek, typically a startling and slurred phee-phee-phee.
Subspecies Of the following four subspecies described in the literature, only the first two (blakistoni & doerriesi) are currently accepted by science. The other two (karafutonis and piscivorus) were likely specimens of blakistoni and doerriesi, respectively, and are presented here only for historical interest. The divergence of the Japanese birds of the nominate subspecies and the Russian subspecies was found to be extreme based on phylogeography, with a point of divergence of at least a half a million years.Omote, K., Surmach, S. G., Kohyama, T. I., Takenaka, T., Nishida, C., & Masuda, R. (2018). Phylogeography of continental and island populations Of Blakiston's Fish-Owl (Bubo blakistoni) in Northeastern Asia. Journal of Raptor Research, 52(1), 31–41.
K. b. blakistoni (Seebohm, 1884). Hokkaido, N. Japan and Kuriles. Lores of facial disc tawny-brown with narrow black shaft-stripes; above eyes, around bill base and on forehead a row small, stiff almost completely white feathers; chin largely white. Rest of head and underparts brown with blackish-brown shaft-stripes and buff feather tips; back is darker. The mantle is somewhat lighter and more rufous and with blackish-brown bars as well as dark brown shaft-streaks. Wings deep brown with numerous buff yellow bars. Tail dark brown with 7–8 cream-yellow bars. Underparts light buff-brown with blackish-brown shaft streaks and narrow light brown wavy cross-bars. The wing chord measures , the tail measures and the tarsus measures .