Blakiston's Fish Owl
|Blakiston's Fish Owl|
|Genus:||Bubo (but see text)|
Blakiston's Fish Owl, Bubo blakistoni, is a fish owl, a sub-group of eagle owls who specialized in hunting riparian areas. 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 related species were previously placed in the genus Ketupa; mtDNA cytochrome b sequence data is equivocal on which genus name is applied for this species. Its habitat is riparian forest, with large, old trees for nest-sites, near lakes, rivers, springs and shoals that don't 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.
It is more correct to call this species the Blakiston's Eagle Owl. This is because it is more closely related to the species of the main subgenus of the species, Bubo Dumeril, than the subgenus of fish owls that it was believed to be more close to, B. Ketupu. This was proven by osteological (skeleton/bone-related) and DNA-based tests in 2003 by ornithologists/taxonomists Prof. Dr. Michael Wink and Dr. Claus König, author of Owls of the World.
Blakiston's Fish Owl is the largest living species of owl. A field study of the species showed males weighing from 3.15 to 3.45 kg (6.9 to 7.6 lb), while the female, at up to 3.36 to 4.6 kg (7.4 to 10.1 lb), is about 25% larger. Blakiston's Fish Owl measures 60–72 cm (24–28 in) in total length, and thus measures slightly less at average and maximum length than the Great Gray 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 races 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 30 mm (1.2 in) 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. However, the average measurements of Blakiston's Fish Owl surpass the average measurements of the Eurasian Eagle-Owl in the three major categories: weight, length, and wingspan, making Blakiston's the overall largest species of owl. The maximum wingspan of the Blakiston's Fish Owl is also greater than any known Eagle-Owl. The Blakiston's is noticeably larger than the other three extant species of fish owl. Among standard measurements, which at average and maximum are greater than any other living owl other than tail length, the wing chord measures 44.7–56 cm (17.6–22.0 in), the tail measures 26.5–29 cm (10.4–11.4 in), the tarsus is 7.3–9.5 cm (2.9–3.7 in) and the culmen is around 7.1 cm (2.8 in).
Superficially, this owl somewhat resembles the Eurasian Eagle-Owl but is paler and has relatively broad and ragged ear tufts which hang slightly to the side. The upperparts are buff-brown and heavily streaked with darker brown coloration. The underparts are a paler buffish-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.
Vocalizations differ among the recognized subspecies. In the Japanese subspecies, 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, HOOO-hoooo (here, the male call is in capital letters (HOO) and the female call in lower case (hoo)). This duet is so synchronized that those unfamiliar with the call often think it is only one bird calling. When an individual bird calls, it sounds like hoo-hooo. Juveniles have a characteristic shriek.
Of the following four subspecies described in the literature, only the first two (B. b. blakistoni & B. b. doerriesi) are currently accepted by science. The other two (B. b. karafutonis and B. b. piscivorus) were likely specimens of B. b. blakistoni & B. b. doerriesi, respectively, and are presented here only for historical interest.
- B. 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 buffy-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.
- B. b. doerriesi (Seebohm, 1884). E. Siberia south to Vladivostok region and Korean border area. Larger than nominate with large white patch on top of the head; tail less marked and bars incomplete.
- B. b. karafutonis (Kuroda, 1931). Sakhalin. Smaller than nominate race and darker, especially on back and ear-coverts; tail with narrower dark brown bars and the light bars more numerous (8-9 against 7 in nominate).
- B. b. piscivorus (Meise, 1933). W. Manchuria. Paler overall than doerriesi, ground color of underparts grayish-white (not buff-brown); tail-bars not fully creamy-yellow, central rectrices having white inner webs almost to base; chin pure white.
Blakiston's Fish Owl occurs in dense old-growth forest near waterways or wooded coastlines. The species requires cavernous old-growth tree cavities for suitable nest sites and stretches of productive rivers that remain at least partially unfrozen in winter. In the frigid northern winters, open water is found only where the current is sufficiently fast-flowing or there is an upwelling of warm spring water. Slower-moving streams are equally likely to support these owls as the main river channels and they only need a few meters of open water to survive a winter.
Feeding and behavior
The Blakiston's Fish Owl feeds on a variety of aquatic prey. The main one is fish, with common prey including pike (Esox reichertii), catfish, trout and salmon (Oncorhynchus). Some fish these owls catch are quite large. Jonathan Slaght estimated that some fish caught are up to two to three times their own weight and has seen owls keep one foot on a tree root to be able to haul a large catch onto a bank. In Russia, amphibians are taken in great quantity in spring, especially Dybowski's frog (Rana dybowskii). Crayfish (Cambaroides) and other crustaceans are known to be taken in some numbers, but the extent of their importance in the Blakiston's fish owl's diet is unknown.
A wide variety of mammalian prey are described from Japan, including martens (Martes) and rodents. Unidentified bats have turned up occasionally in Blakiston's Fish Owl pellets in the Russian Far East, although were much more prominent in the diet of Eurasian Eagle-Owls there (79 Eagle-Owl pellets and 10 Fish Owl pellets had bat remains, respectively).
Large mammals are sometimes taken by this species, including hares, rabbits, fox, cats (Felis catus) and small dogs (Canis lupus familiaris). Fewer records are known of bird predation, but they are known to capture avian prey to the size of hazel grouse (Tetrastes bonasia) and a variety of waterfowl species.
The two most common hunting methods for Blakiston's Fish Owl are wading through river shallows and perching on the river bank and waiting for movement in the water. In this behavior, an individual may wait for four hours until it detects prey and the species is perhaps most often witnessed while hunting in this method. Upon identifying prey, fish owls either drop directly into the shallow water or sail a short distance. It also takes carrion, as evidenced by fish owls in Russia being trapped in snares set for furbearing mammals, which use raw meat as bait. While small prey such as frogs and crayfish are taken back to an habitual perch for immediate consumption, larger prey such as fish and waterfowl are dragged onto a bank and finished off before being flown off with.
These owls are primarily active at dusk and dawn. During the brood-rearing season, these owls are likely to be seen actively hunting or brooding during the day. For an owl, it spends unusual amounts of time on the ground. Occasionally, an owl may even trample out a regular foot path along riverbanks it uses for hunting. Early reports of concentrations of as many as 5-6 owls near rapids and non-freezing springs are dubious, as these owls are highly territorial.
This bird does not breed every year due to fluctuations in food supply and conditions. Courting occurs in January or February. Laying of eggs begins as early as mid-March, when ground and trees are still covered with snow. These owls prefer nesting in hollow tree cavities in Japan and Russia. In Russia, trees selected for nesting can consist of elm (Ulmus), Japanese poplar (Populus maximowiczii), willow (Salix), chosenia (Chosenia arbutifolia), Mongolian oak (Quercus mongolica), ash (Sorbus), and stone birch (Betula ermanii). Nest height range is 2–18 m (6 ft 7 in–59 ft 1 in). Reports of nesting on fallen tree trunks and on the forest floor are very rare occurrences at best and possibly untrue. Other than nest cavities, there are very isolated records of nesting on cliff shelves and in old black kite nests.
Nest cavities have to be quite large in order to accommodate these birds. Clutch size is 1 to 3, usually 2. In Russia, clutches are usually just one egg. Eggs are 6.2 cm (2.5 in) long and 4.9 cm (1.9 in) wide. The males provide food for the incubating female and later the nestlings. The incubation period is about 35 days and young leave the nest within 35–40 days but are often fed and cared for by their parents for several more months. Data on breeding success are scant: on Kunashir Island during a six-year period breeding success was 24%; with six fledglings resulting from 25 eggs. Juveniles linger on their parents' territory for up to two years before dispersing to find their own. Blakiston's fish owls can form pair bonds as early as their second year and reach sexual maturity by age three. This unusually long pre-dispersal period may be why this owl is occasionally reported as gregarious, as sets of parents and juveniles will congregate but not unrelated owls.
Once full-sized, these owls have few natural predators. However, they may be more vulnerable to attack from mammalian carnivores since, unlike other eagle owls which typically perch and hunt from trees or inaccessible rock formations, they hunt mainly on the ground along riverbanks. There are two records of natural predation on adults from Russia and none in Japan. The first is a record of an adult fish owl being stalked and killed by a Eurasian lynx (Lynx lynx), while the owl hunted along a river bank. Other was another adult which was similarly ambushed by an Asian black bear (Ursus thibetanus).
The Blakiston's Fish Owl is endangered due to the widespread loss of riverine forest, increasing development along rivers and dam construction. The current population in Japan is approximately 100-150 birds (20 breeding pairs and unpaired individuals), whereas on mainland Asia the population is higher, variously estimated at several hundred or perhaps thousands of individuals. In Russia, fish owls are killed by fur-trappers (see above), drown in nets set for salmon, and are shot by hunters. In Japan, death by hunting is unlikely, but fish owls have been hit by cars and killed by powerlines. Local conservation efforts in Japan have been undertaken including education and installation of large nest-boxes.
Importance to indigenous peoples
Blakiston's fish owl is revered by the Ainu peoples of Hokkaido, Japan as a Kamuy (divine being) called Kotan koru Kamuy (God that Protects the Village). In Russia, the species is considered a food source by the Evens people in northern Siberia and the northern Russian Far East. Previously, fish owls were hunted as a food source by the Udege peoples in Primorye and were favored due to their high fat content. However, this practice has locally fallen out of favor in recent times.
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- Angier, Natalie (2013-02-25). "The Owl Comes Into Its Own". The New York Times. Retrieved 2013-02-27. "Nearly a yard high, weighing up to 10 pounds and with a wingspan of six feet, Blakiston’s is the world’s largest owl..."
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