Steller's Sea Eagle
The Steller's Sea Eagle, Haliaeetus pelagicus, is a large bird of prey in the family Accipitridae. It lives in coastal northeastern Asia and mainly preys on fish. On average, it is the heaviest eagle in the world, at about 5 to 9 kilograms (11 to 20 lb; 0.79 to 1.4 st), but may lag behind the Harpy Eagle (Harpia harpyja) and Philippine Eagle (Pithecophaga jefferyi) in some standard measurements. This bird is named after the German naturalist Georg Wilhelm Steller.
This species was first described as Aquila pelagica by Peter Simon Pallas, in either 1811 or 1826 depending on the source. Subsequently, many generic and specific names have been variously spelled, e.g., Haliaetus pelagicus, Haliaetos pelagica, Faico leucopterus, Faico imperator, Thalassaetus pelagicus, Thalassaetus macrurus, Haliaeetus macrurus and most recently Thallasoaetus pelagicus. Besides its normal common name, the species has sometimes been referred to as the Pacific Eagle or White-shouldered Eagle. In Russian, the eagle has been called morskoi orel (sea eagle), pestryi morskoi orel (mottled sea eagle) or beloplechii orlan (white-shouldered eagle). In Japanese, it is called 0-washi (large eagle or great eagle).
Steller's Sea-eagle is the biggest bird in the genus Haliaeetus and is one of the largest raptors overall. Females typically weigh from 6.8 to 9 kilograms (15 to 20 lb; 1.07 to 1.4 st), while males are rather lighter with a weight range from 4.9 to 6 kilograms (11 to 13 lb; 0.77 to 0.94 st). At its average weight, the Steller's outweighs both the average Harpy and the average Philippine Eagles by over 0.5 kilograms (1.1 lb; 0.079 st). The Steller's Sea Eagle's length can range from 85 to 105 cm (33 to 41 in). The wingspan is from 1.95 to 2.5 m (6.4 to 8.2 ft) and the wing chord measurement is 57–68 cm (22–27 in). The Steller's sea eagle has the second largest median wingspan of any eagle. Both the wing chord and wingspan, at an average of 2.13 m (7.0 ft), are similar or slightly smaller than to those of the Steller's close relative, the White-tailed Eagle (Haliaeetus albicilla), which is rather smaller in both weight and total length. As in most Haliaeetus eagles, the tarsus and the tail in this species are relatively short compared to other very large eagles at 9.5–10 cm (3.7–3.9 in) and 32–34.5 cm (13–13.6 in) in length, respectively. The bill is very large. In fact, the skull (at around 14.6 cm (5.7 in)) and the culmen (at around 7 cm (2.8 in)) of the Steller's Sea Eagle are the largest of any eagle and are comparable in size to those of the largest accipitrids, the Old World vultures.
The Steller's Sea Eagle is dark brown to black over the majority of its body, with white on the shoulders, thigh and crown. They have wedge-shaped, white tails. The bold, pied coloration of adults may play some part in social hierarchies with other eagles of their own species during the non-breeding season, although this has not been extensively studied. The eyes, the bill and the feet of adults are all yellow in coloration. Two subspecies have been named: The relatively widespread nominate pelagicus and the virtually unknown H. p. niger. The latter name was given to the population which lacked white feathers except for the tail and supposedly was resident all year in Korea. Last seen in 1968 and long believed to be extinct, a female matching H. p. niger in appearance was born in captivity in 2001. Both its parent were "normal" in appearance, indicating that H. p. niger is an extremely rare morph rather than a valid subspecies, as had already been suggested earlier.
Steller's Sea Eagles are not extensively known for their voices but are known to make a deep barking cry, ra-ra-ra-raurau, in aggressive interactions. During their breeding season, they have been heard to make calls to each that sound like very loud gulls.
The first down plumage of new nestlings is silky white, though they soon turn a smoky brown-gray. As in other sea eagles, remiges and retrices of the first-year plumage are longer than adults. Juvenile plumage is uniformly dark brown with white feather bases and light mottling on the retrices. The young Steller's Sea Eagle has a dark brown iris, whitish legs and blackish-brown beak. Through at least three intermediate plumages, mottling in the tail decreases, body and wing feathering acquires a bronze cast, and the eye and bill lighten in color. Definitive plumage is probably reached in the fifth year of life, based on fragmentary data from captives. First and intermediate plumages are difficult to distinguish from those of the White-tailed Eagle, which occurs in the entire breeding range of the Steller's.
Systematics and taxonomic status
The relationships of Steller's Sea-eagle are not completely resolved. mtDNA cytochrome b sequence data tentatively suggests that this species's ancestors diverged early in the colonization of the Holarctic by sea eagles. This is strongly supported by morphological traits such as the yellow eyes, beak, and talons shared by this species and the other northern sea-eagles, the White-tailed and Bald Eagles (Haliaeetus leucocephalus), and biogeography. It is unique among all sea eagles in having a yellow bill even in juvenile birds, and possessing 14, not 12, rectrices.
Distribution and habitat
Steller's Sea Eagle breeds on the Kamchatka Peninsula, the coastal area around the Sea of Okhotsk, the lower reaches of the Amur river and on northern Sakhalin and the Shantar Islands, Russia. The majority of birds winter farther south, in the southern Kuril islands, Russia and Hokkaidō, Japan. That being said, the Steller's Sea-eagle is less vagrant than the White-tailed Eagle, usually lacking the long-range dispersal common in juveniles of that species.
The large body size (see also Bergmann's Rule) suggests that it is a glacial relict, meaning that it evolved in a narrow subarctic zone of the northeasternmost Asian coasts, which shifted its latitude according to ice age cycles, and never occurred anywhere else.
This bird nests in two habitats: along sea coasts and alongside large rivers with mature trees. They nest on large, rocky outcroppings or at the tops of large trees. Usually areas with large Erman's birches and floodplain forests of larches, alders, willows and poplar seem to be the preferred nesting spots. Some eagles, especially those that nest in sea coast, may not migrate. The timing, duration and extent of migration depends on ice conditions and food availability. On Kamchatka, eagles overwinter in forests and river valleys near the coast, but are irregularly distributed over the peninsula. Most wintering birds there appear to be residential adults. Steller's sea eagles that do migrate fly down to winter in rivers and wetlands in Japan but will occasionally move to mountainous inland areas as opposed to the sea coast. Each winter, drifting ice on the Sea of Okhotsk drives thousands of eagles south. Ice reaches Hokkaido in late January. Eagle numbers peak in the Nemuro Strait in late February. On Hokkaido, eagles concentrate in coastal areas and on lakes near the coast, along with substantial numbers of White-tailed Eagles. Eagles depart between late March and late April, adults typically leaving before immatures. Migrants tend to follow sea coasts and are usually observed flying singly. In groups, migrants are typically observed flying 100–200 m (330–660 ft) apart. On Kamchatka, most migrants are birds in transitional plumages. They are also occasionally seen flying over the Northern ocean or perching on sea ice during the winter.
The Steller's Sea-eagle mainly feeds on fish. Their favored prey is salmon (Onchorhynchus spp.) and trout. Like most Haliaeetus eagles, they hunt fish almost exclusively in shallow water. Relatively large numbers of these normally solitary birds can be seen congregating on particularly productive spawning rivers in August through September due to an abundant food supply. On Kamchatka, aggregations of as many as 700 eagles have been reported, though this is probably impossible today and much smaller groups are the norm. In summer, live fish, typically in the range of 20 to 30 cm (7.9 to 12 in) in length, are fed to the young at the nest. Typically the parents will catch about 2 or 3 fish for the young to eat. In autumn, when many salmon die after spawning, dead fish may be consumed more often than live ones. On Hokkaido, eagles are attracted by abundant Pacific cod (Gadus macrocephalus) which peak in the Rausu Sea and the Nemuro Straits in February. This resource supports an important commercial fishery which in turn helps to support eagles. Alaska pollock (Theragra chalcogramma), along with the cod, is the most important food source for wintering eagles in Japan.
Fish comprise about 80% of the diet of eagles nesting in the Amur River. Elsewhere, other prey can comprise almost an equally important portion of the diet. Non-piscivore prey is comprised largely of water-dwelling birds, including ducks, geese, swans, cranes, herons and gulls. Along the sea coast and in Kamchatka, water birds are the most common prey for Steller's Sea Eagles. Among bird prey, this eagle has shown a strong local preference for Slaty-backed Gulls (Larus schistisagus). In Russia, among upland bird species, Black-billed Capercaillie (Tetrao parvirostris) can be an important prey species. In one case, a Steller's sea eagle was observed feeding on a rare prey item in the Northern Hemisphere, a great albatross (Diomedea), as that genus nests in the sub-Antarctic oceans. This sea eagle may supplement its diet with various mammals (especially hares), crabs, mussels and squid when given the opportunity. Carrion, especially that of mammals, is readily eaten during the winter. Around 35% of eagles wintering in Japan move inland and feed largely on mammalian carcasses, predominantly Sika Deer (Cervus nippon). This eagle has been recorded preying occasionally on young seals. It was estimated in one study (Brown & Amaden), that some seal pups carried off in flight by the eagles weighed at least 9.1 kg (20 lbs), which (if true) would be the greatest load carrying ever known for a bird; however the prey weights were not verified. Often seals and sea lion of any size are eaten as carrion and, using the huge bill, may be dismembered where found rather than flown with.
Most often, Steller's Sea Eagles hunt from a perch in a tree or rocky ledge located 5–30 m (16–98 ft) above the water. When prey is spotted, the bird dives from its perch. Eagles may also hunt on the wing, while circling 6–7 m (20–23 ft) above the water. Again, prey is captured by diving. Eagles sometimes hunt by standing in or near shallow water on a sandbank, spit, or ice-flow, grabbing passing fish. Where feeding occurs in groups, kleptoparasitism is common. Kleptoparasitism is most beneficial in procuring food during periods of food abundance and in large feeding aggregations. Immatures use kleptoparasitism as much as adults, but are attacked more often by adults than birds of similar age. Adults appear to benefit most from this behavior. The bold color patterns of adults may be an important signal influencing the formation of feeding groups. Outside the breeding period, these eagles probably roost communally near their feeding sites. When salmon and trout are spawning, feeding groups of Steller's Sea Eagles may mix with smaller Golden Eagles (Aquila chrysaetos) and White-tailed Eagles. This area is the only one in the Golden Eagle's nearly circumpolar range where they are extensively dependent on fish for prey. Kleptoparasitism is sometimes recorded within the species. Occasionally, the smaller species may steal a fish away from the Steller's, especially if it is distracted by aggression from conspecifics. However, in several cases, the Steller's has been observed to come away with the prey after using its superior size to dominate. In other cases, though, the three eagle species have been observed to feed in close proximity and seem to be outwardly indifferent to each other's presence. In inland areas, where Golden, Bald and White-tailed Eagles compete over food sources which are not as abundant as spawning salmonids, aggressive interspecies competition can be more common.
This eagle builds several aeries, being bulky constructions of twigs and sticks, at a height of up to 150 cm (59 in) and diameter of up to 250 cm (98 in). They usually place such nests high up on trees and rock at 15 to 20 m (49 to 66 ft) above the ground. Alternate nests are usually built within 900 m (3,000 ft) of each other. In one case, two active nests were found to have been located within 100 m (330 ft).
After courtship, which usually occurs between February and March, the animals may build the nest and copulate on it. They lay their first white-green eggs around April to May, clutches can contain from one to three eggs, with two being the average. Usually only one chick survives to adulthood. After an incubation period of around 39 – 45 days the chicks hatch, the helpless, whitish-down covered young are born. Incubation begins with the first hatching, which occurs in mid-May through late June. The eaglets fledge in August or early September.
Eggs and very small nestlings can be preyed on by arboreal mammals, such as sables and ermine, and birds, usually corvids. Any of these small, clever nest predators rely on distraction and stealth to predate the eagle's nests and are killed if caught by either of the parents. Once it reaches roughly adult size in the fledging stage, few predators can threaten this species. In one case, a brown bear (Ursus arctos) was able to access a nest located on a rock formation and ate a fledging eaglet, though this is believed to be exceptional. Fully-grown fledgings in tree nests are probably invulnerable to predation, as there are no large mammalian carnivores which can climb trees in the species' range. Due primarily to egg predation and nest collapses, only 45–67% of eggs are successfully reared to adulthood and up to 25% of nestlings may be lost. However, once fully grown, the eagle has no natural predators.
This species is classified as Vulnerable by the IUCN. They are legally protected, being classified as a National Treasure in Japan and mostly occurring in protected areas in Russia. However, many threats to their survival persist. These mainly include habitat alteration, industrial pollution and over-fishing, which in turn decreases their prey source. The current population is estimated at 5,000 and decreasing. It was observed that recent heavy flooding, which may have been an effect of global climate change, caused almost complete nesting failure for the eagles nesting in Russian rivers due to completely hampering the ability of the parents to capture the fish essential to their nestlings' survival. Persecution of the bird in Russia continues, due to its habit of stealing furbearers from trappers. Due to a lack of other accessible prey in some areas, increasingly eagles on Hokkaido have moved inland and scavenged on sika deer carcasses left by hunters, exposing them to a risk of lead poisoning through ingestion of lead shot.
In Kamchatka, 320 pairs have been recorded. An additional 89 nesting areas are not monitored. In the mountains of Koryakan and along the Bay of Penshina, over 1,200 pairs breed and at least 1,400 juveniles occur. About 500 pairs live in the Khabarovsk region of the Okhostsk coast, and 100 on the Shantar Islands. Another 600 pairs occur in the lower Amur. There are 280 pairs on Sakhalin Island and a few on the Kurile Islands. The total population is approximately 3,200 breeding pairs. Possibly up to 3,500 birds winter on Kamchatka, and another approximate 2,000 may occur on Hokkaido.
- IUCN redlist.
- Etymology: Haliaeetus, New Latin for "sea-eagle". pelagicus, "of the open seas", from Ancient Greek pelagos, the ocean.
- Ferguson-Lees & Christie (2001). Raptors of the World. ISBN 0-7136-8026-1
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- Etymology: niger, Latin for "black".
- Kaiser, M. (2010). A living specimen of the dark form of Steller’s Sea Eagle, Haliaeetus pelagicus ("niger") in captivity. J. Orn. Online. doi:10.1007/s10336-010-0580-2
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- Ueta, M.; McGrady, M. J.; Nakagawa, H.; Sato, F.; Masterov, V. B. 2003. Seasonal change in habitat use in Steller's sea eagles. Oryx 37: 110-124.
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- Photo Keywords : steller's sea eagle : White-tailed Eagle and Steller's Sea Eagle. Golden First Light (2012-06-21). Retrieved on 2012-08-21.
- McGrady, M.J., E. Potapov, and I. Utekhina. 1999. Brown Bear (Ursus arctos) feeds on Steller's Sea-eagle (Haliaeetus pelagicus) nestling. Journal of Raptor Research 33:342–343.
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- Cited works
- Brown, Leslie Hilton (1976): Eagles of the world. David & Charles, Newton Abbot. ISBN 0-7153-7269-6
- Ferguson-Lees, James; Christie, David A. & Franklin, Kim (2005): Raptors of the world: a field guide. Christopher Helm, London & Princeton. ISBN 0-7136-6957-8
- True, Dan (1980): A family of eagles. Everest, New York. ISBN 0-89696-078-1
- Wink, M.; Heidrich, P.; Fentzloff, C. (1996). "A mtDNA phylogeny of sea eagles (genus Haliaeetus) based on nucleotide sequences of the cytochrome b gene" (PDF). Biochemical Systematics and Ecology 24 (7–8): 783–791. doi:10.1016/S0305-1978(96)00049-X. http://www.uni-heidelberg.de/institute/fak14/ipmb/phazb/pubwink/1996/20_1996.pdf.