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Riesenseeadler Walsrode 2014 01

Steller's sea eagle (Haliaeetus pelagicus) in Weltvogelpark Walsrode.

Steller's sea eagle (Haliaeetus pelagicus) is one of the largest of the sea and fish eagles of the genus Haliaeetus. These large blackish-brown birds have an enormous, strongly arched yellow bill. The feathers on the shoulders, tail and legs are white, and females are generally the larger sex. On average, it is the heaviest eagle in the world, at about 5 to 9 kg (11 to 20 lb), but may be below the harpy eagle (Harpia harpyja) and Philippine eagle (Pithecophaga jefferyi) in some standard measurements. It is named after the German naturalist Georg Wilhelm Steller.

Biology Edit

Steller's sea eagles start to build their large, bulky nests in the trees in late February and early March (3). The first eggs are laid in mid-April, and clutch size varies from one to three eggs; hatchlings emerge in mid-May to mid-June and begin to fly by August and early September .

These large birds feed predominately on salmon (Onchorhynchus spp.), which are taken both dead and alive. Prey is usually caught by swooping from perches located at the waters' edge, or from circling and diving down; occasionally birds will stand in the shallows to catch fish. Steller's sea eagles have a large, powerful bill that is perfectly adapted to ripping and tearing at flesh and these birds will also prey on other fish and the carcasses of animals such as seals and sea lions. Where there are large congregations of prey such as salmon, groups of eagles will gather and individuals will often attempt to steal food from each other in a behavior known as 'kleptoparasitism'.

Range and Habitat Edit

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 Asiancoasts, 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.

Reproduction Edit

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 treesand 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 Asiatic 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.

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