Steller's sea eagle

Etymology

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 ō-washi (large eagle or great eagle).

Size

Steller's sea eagle is the biggest bird in the genus Haliaeetus and is one of the largest raptors overall. Females vary in weight from 6,195 to 9,500 g (13.658 to 20.944 lb), while males being rather lighter with a weight range of 4,900 to 6,800 g (10.8 to 15.0 lb). The average weight is variable, possibly due to seasonal variation in food access or general condition of eagles, but has been reported as high as a mean mass of 7,757 g (17.101 lb) to a median estimate weight of 6,250 g (13.78 lb), excluding expired eagles that were poisoned by lead and endured precipitous weight loss by the occasion of their deaths. At its average weight, the Steller's seems to outweigh the average harpy by approximately 500 g (1.1 lb) and the average Philippine eagles by more than 1,000 g (2.2 lb). Steller's sea eagle can range in total length from 85 to 105 cm (2 ft 9 in to 3 ft 5 in), apparently males average about 89 cm (2 ft 11 in) in length, while females average about 100 cm (3 ft 3 in), marginally shorter on average than the harpy eagle and about 65 mm (2.6 in) shorter than the Philippine eagle. The wingspan is from 1.95 to 2.5 m (6 ft 5 in to 8 ft 2 in) and the wing chord measurement is 560 to 680 mm (22 to 27 in). The sea eagle's wingspan is one of the largest of any living eagle, at a median of 2.13 m (7 ft 0 in) per Ferguson-Lees (2001) or a median of 2.2 m (7 ft 3 in) per Saito (2009). Closest are the closely related white-tailed eagle (Haliaeetus albicilla), at reported median wingspans of 2.1 and 2.18 m (6 ft 11 in and 7 ft 2 in) and the unrelated wedge-tailed eagle (Aquila audax), at reported average wingspans of 2.04 and 2.23 m (6 ft 8 in and 7 ft 4 in); nonetheless, both other eagles are rather smaller in overall size, particularly body mass. The Steller's sea eagle's absolute maximum wingspan is less certain; many sources place it at up to 2.45 m (8 ft 2 in). However, less substantiated records indicate that it may also reach up to 2.74 m (9 ft), if true this would make it one of the largest eagles in the world by wingspan as well as body size.

Standard measurements and physiology

As in most Haliaeetus eagles, the tarsus and tail are relatively short compared to other very large eagles at 95–100 mm (3.7–3.9 in) and 320–390 mm (13–15 in) in length, respectively, the Philippine eagle besting it by up to 40 mm (1.6 in) and 110 mm (4.3 in) apparently. In all sea and fish eagles, the toes are relatively short and stout, with the bottom of the foot covered in spiracles and the talons being relatively shorter and more strongly curved than in comparably sized eagles of forests and fields, such as the "booted eagle" group (i.e. Aquila) or "harpy eagles", all of these specializations developed in the aid of capturing fish rather than medium-sized mammals and large birds, although clearly these are not excluded from capture. As in all fish and sea eagles, as well as the majority of the world's fish-eating raptors, Steller's sea eagle has spiracles, which are bumpy waves all along the bottom of their feet, which allow them to hold fish that may otherwise slip out of their grasp. The feet are very powerful despite not bearing talons as long as those of a harpy eagle. In one case, a wildlife veteranian was badly injured when a female eagle grabbed his arm and embedded her talons, piercing through to the other side of his arm. Perhaps the most noted physical feature of Steller's sea eagle, other than its overall great size, is its extremely large bill and prominent head. The skull is around 14.6 cm (5.7 in) in total length, the culmen is from 62 to 75 mm (2.4 to 3.0 in) and the bill from the gape to the tip is around 117 mm (4.6 in). Steller's sea eagle's bill is probably the largest of any living eagle, just surpassing to the Philippine eagle with a sole known culmen measurement (from a mature female) of 72.2 mm (2.84 in), and are similar in robustness (if slightly shorter in culmen length) to those of the largest accipitrids, the Old World vultures.

Coloring

The mature Steller's sea eagle is dark brown to black over the majority of its body, with strongly contrasting white on the lesser and median upper-wing coverts, underwing coverts, thighs, under-tail coverts and tail. Their diamond-shaped, white tails are relatively longer than those of the white-tailed eagle. The bold, pied coloration of adults may play some part in social hierarchies with other eagles of their own species during the nonbreeding season, although this has not been extensively studied. The eyes, the bill, and the feet of adults are all yellow in colouration. Two subspecies have been named: The relatively widespread nominate H. p. 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 hatched in captivity in 2001. Both its parents were "normal" in appearance, indicating that H. p. niger is an extremely rare morph rather than a valid subspecies, as had been suggested earlier.

The first downy plumage of new nestlings is silky white, though it soon turns a smoky brown-grey. As in other sea eagles, remiges and rectrices of the first-year plumage are longer than those of adults. Juvenile plumage is largely a uniform dark brown with occasional grey-brown streaking about the head and the neck, white feather bases, and light mottling on the rectrices. The tail of the immature eagle is white with black mottling distally. 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 colour. 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 co-occurs in the entire breeding range of the Steller's.

Voice

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. Their call is similar to the white-tailed eagles but deeper. During the display at the beginning of the breeding season, they have been heard to make calls to each that sound like very loud, deep-voiced gulls.

Systematics and taxonomic status

The relationships of Steller's sea eagle are not completely resolved. mtDNA cytochrome b sequence data tentatively suggest that this species's ancestors diverged early in the colonization of the Holarctic by sea eagles. This is strongly supported by biogeography and by morphological traits such as the yellow eyes, beak, and talons shared with the other northern sea eagles, the white-tailed and bald eagles (Haliaeetus albicilla and H. leucocephalus). 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, Steller's sea eagle is less vagrant than the white-tailed eagle, usually lacking the long-range dispersal common in juveniles of that species. Vagrant eagles have been found in North America, at locations including the Pribilof Islands and Kodiak Island, inland to as far as Peking in China and Yakutsk in Russia's Sakha Republic, and south to as far as Taiwan, but these are considered to be individual eagles that have strayed far from the species' typical range.

The large body size (see also Bergmann's rule) and distribution of Steller's sea eagle suggests it is a glacial relict, meaning 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 (Betula ermanii) 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.

Diet

Steller's sea eagle mainly feeds on fish. Their favored prey in river habitats are salmon (Oncorhynchus spp.) and trout. Among these, pink salmon (O. gorbuscha) and chum salmon (O. keta) are reportedly favored, sometimes intensely supplemented by grayling (Thymallus thymallus) and three-spined stickleback (Gastrossteus aculeatus). While pink and chum salmon average approximately 2,200 and 5,000 g (4.9 and 11.0 lb) in mature mass, respectively, Steller's sea eagle not infrequently preys on fish up to 6,000 to 7,000 g (13 to 15 lb). In coastal areas, nesting eagles may feed on Bering wolffish (Anarchichas orientalis), Hemitripterus villosus, Aptocyclus ventricosus and Myoxocephalus spp. 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 much smaller groups are the norm. In summer, live fish, typically in the range of 20 to 30 cm (7.9 to 11.8 in) in length, are fed to the young at the nest. Normally, the parents catch about two or three fish for the young to eat each day. In autumn, when many salmon die after spawning, dead fish tend to be consumed more often than live ones, and these are the main food for Steller's sea eagles that overwinter in inland rivers with unfrozen waters. 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. These eagles may walk boldly within a few feet of fishermen when both are capturing fish during winter, but only familiar ones they have encountered previously: they behave warily and keep their distance if strangers are present.

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. Nonpiscine prey consists 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). Common and thick-billed murres (Uria aalge and U. lomvia, respectively) dominated the diet around the Sea of Okhotsk, followed by black-legged kittiwakes (Rissa tridactyla), slaty-backed gulls, crested auklets (Aethia cristatella), and pelagic cormorants (Phalacrocorax capillatus). Small chicks of murres and cormorants were sometimes taken alive in Russia and brought back to nests, where they independently fed on remains of fish in the eagles' nests until they were killed themselves. In Russia, upland bird species, black-billed capercaillie (Tetrao parvirostris) and willow and rock ptarmigan (Lagopus lagopus & L. muta) can be an important prey species. Grouse are not typically taken by other Haliaeetus species. Other landbirds hunted by Steller's sea eagles have included short-eared owls (Asio flammeus), snowy owl (Bubo scandiaca), carrion crow (Corvus corone) and common raven (Corvus corax), as well as (rarely) smaller passerines. 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, Nereis worms and squid when given the opportunity.

Mammalian carnivores are apparently readily hunted. Those recorded as prey have included sable (Martes zibellina), American mink (Neovison vison), Arctic fox (Vulpes lagopus), red fox (Vulpes vulpes), and small domestic dogs (Canis lupus domesticus). Smaller mammals have also been recorded as prey, including northern red-backed vole (Clethrionomys rutilus) and tundra vole (Microtus oeconomus). 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). In winter, immature Steller's sea eagles may frequent slaughterhouses to pirate bits of offal. This eagle has been recorded preying occasionally on young seals. It was estimated in one study (Brown & Amadon), that some seal pups carried off in flight by the eagles weighed at least 9.1 kg (20 lb), 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. It is reported that, compared to its white-tailed and bald eagle relatives, Steller's sea eagle is a more "aggressive, powerful, and active" raptor. 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. However, a video from Russia shows a juvenile Steller's sea eagle aggressively displacing an adult from food during a protracted battle. Outside the breeding period, these eagles probably roost communally near their feeding sites. When salmon and trout are dying in winter after their summer spawning, feeding groups of Steller's sea eagles may mix with smaller golden eagles (Aquila chrysaetos) and white-tailed eagles to exploit this food source. 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, and both juvenile and adult Steller's may lose fish to the smaller species even face-to-face, especially a less assertive bird, such as immature Steller's. One video shows a golden eagle engaging an immature Steller's in a conflict and ultimately displacing it after maintaining a superior grip despite its smaller size. In other cases, the Steller's have been photographed coming away with the prey after using its superior size to dominate, usually by bearing down its mass and large bill over the smaller eagles. 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 these fish and, more importantly, compete for nesting ranges, aggressive interspecies competition can be more common. White-tailed eagles and golden eagles have even killed one another in Scotland, in cases of competition for abutting nesting ranges. As in many sea and fish eagles, Steller's sea eagle may attempt to steal (and occasionally succeed in procuring) fish from osprey (Pandion haliaetus) where they coexist. In one case, a cinereous vulture (Aegypius monachus), the largest living accipitrid, was observed to be pursued in flight and kleptoparasitized by a Steller's sea eagle.

Reproduction

This eagle builds several aeries, being bulky constructions of twigs and sticks, at a height up to 150 cm (59 in) and diameter 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, sometimes in trees up to 45 m (148 ft). 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).

Courtship, which usually occurs between February and March, and reportedly simply consists of a soaring flight above the breeding area. The Steller's sea eagle copulate on the nest after building it. They lay their first greenish-white eggs around April to May. The eggs range from 78 to 85 mm (3.1 to 3.3 in) height and 57.5 to 64.5 mm (2.26 to 2.54 in) in width and weigh around 160 g (5.6 oz), being slightly larger than those of harpy eagles. Clutches can contain from one to three eggs, with two being the average. Usually, only one chick survives to adulthood, though in some cases as many as three will successfully fledge. After an incubation period around 39 – 45 days the chicks hatch, the helpless, whitish-down covered young are hatched. Incubation begins with the first hatching, which occurs in mid-May to late June. The eaglets fledge in August or early September. Adult plumage is attained at four years of age, but first breeding does not typically occur for another year or two.

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 prey on 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. Excluding the Asian black bear (Ursus thibetanus), which has not thus far been recorded as a predator, no other mammalian carnivores are equal to or greater than the eagle's size 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.

Conservation status

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 overfishing, which in turn decrease 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. About 280 pairs are on Sakhalin Island and a few on the Kurile Islands. The total population is around 3,200 breeding pairs. Possibly, up to 3,500 birds winter on Kamchatka, and another roughly 2,000 may occur on Hokkaido. On the whole, the species' outlook is favorable. Outside the breeding range, food bases in the principal wintering areas are so far secure.