The White-bellied sea eagle is a large diurnal bird of prey that breeds and hunts near water, and fish form around half of its diet. This bird has a white head, rump and underparts, and dark or slate-grey back and wings. In flight, the black flight feathers on the wings are easily seen when the bird is viewed from below. The large, hooked bill is a leaden blue-grey with a darker tip, and the eyes are dark brown. The legs and feet are yellow or grey, with long black talons (claws). Like many raptors, the female is slightly larger than the male. Immature birds have brown plumage, which is gradually replaced by white until the age of five or six years.
White-bellied sea eagles are found regularly from Mumbai eastwards in India, Bangladesh, and Sri Lanka in southern Asia, through all of coastal Southeast Asia including Burma, Thailand, Malaysia, Indonesia, Indochina, the main and offshore islands of the Philippines, and southern China including Hong Kong, Hainan, and Fuzhou, eastwards through New Guinea and the Bismarck Archipelago, and Australia. In the northern Solomons, they are restricted to Nissan Island. These birds occur mainly in coastal areas, islands, estuaries but also in large inland water bodies, lakes, rivers, and wetlands. They usually breed near the water with some forest cover or in rocky areas.
White-bellied sea eagles are generally territorial; some birds form permanent pairs that inhabit territories their throughout the year, while others are nomadic. Immature birds are generally dispersive, with many moving over 50 km (31 mi) away from the area they were raised. These birds are diurnal and often seen perched high in a tree, or soaring over waterways and adjacent land. They spend time singly or in pairs. Small groups of White-bellied sea eagles sometimes gather if there is a plentiful source of food such as a carcass or fish offal on a ship. A pair may cooperate to hunt. During hunting the bird prepares for the strike by holding its feet far forward (almost under its chin) and then strikes backwards while simultaneously beating its wings to lift upwards. They often catch a fish by flying low over the water and grasping it in its talons. Generally, only one foot is used to seize prey. White-bellied sea eagles may also dive at a 45-degree angle from their perch and briefly submerge to catch fish near the water surface. These large birds of prey have a loud goose-like honking call which is heard particularly during the breeding season; pairs often honk in unison and often carry on for some time when perched. The male's call is higher-pitched and more rapid than that of the female.
White-bellied sea eagles are opportunistic carnivores and consume a wide variety of animal prey, including carrion. They hunt mainly aquatic animals, such as fish, turtles and sea snakes, but they will also take birds, such as little penguins, Eurasian coots and shearwaters, and mammals (including flying foxes) as well. White-bellied eagles feed on carrion such as dead sheep, birds, and fish found along the waterline, as well as raiding fishing nets and following cane harvesters.
White-bellied sea eagles are monogamous; pairs remain together until one bird dies, after which the surviving bird quickly seeks a new mate. The breeding season varies according to location - it occurs in the dry season in Papua New Guinea, and from June to August in Australia. During this time a pair of White-bellied sea eagles perform skillful displays of flying such as diving, gliding and chasing each other while calling loudly. They may mirror each other, flying 2-3 m (6.6-9.8 ft) apart and copying each other swooping and swerving. They also perform a talon-grappling display where the pair will fly high before one flips upside down and tries to grapple the other's talons with its own. If successful, the two then plunge cartwheeling before separating as they approach the ground. These birds usually choose tall trees or man-made pylons to nest in. The nest is a large deep bowl constructed of sticks and branches and lined with such materials as grass or seaweed. Nests are generally sited in the forks of large trees overlooking bodies of water. Cliffs are also suitable nesting sites, and on islands, nests are sometimes built directly on the ground. The female usually lays a clutch of 2 dull, white, oval eggs that are incubated over 6 weeks before hatching. Eaglets are covered in white down when they emerge from the egg. Although 2 eggs are laid, it is unusual for both chicks to be reared successfully to fledging (leaving the nest). One egg may be infertile, or the second chick may die in the nest. If the first clutch is lost, the parents may attempt a second brood. Eaglets usually fledge when they are 70 to 80 days old, and remain around the parents' territory for up to 6 months or until the following breeding season. White-bellied sea eagles start breeding from around six years of age.
Although not threatened globally, White-bellied sea eagles have declined in parts of southeast Asia such as Thailand, and southeastern Australia. Human disturbance to their habitat is the main threat, both from direct human activity near nests which impacts on breeding success and from the removal of suitable trees for nesting. In Tasmania, these birds are threatened by nest disturbance, loss of suitable nesting habitat, shooting, poisoning, trapping, and collision with power lines and wind turbines, as well as entanglement and environmental pollution. Estuaries are a favored habitat, and these are often subject to environmental disturbance.
According to the IUCN Red List, the total White-bellied sea eagle population size is around 1,000-10,000 individuals, which is around 670-6,700 mature individuals. The population in China has been estimated at around 100-10,000 breeding pairs. Currently, this species is classified as Least Concern (LC) on the IUCN Red List and its numbers today are decreasing.
Being apex predators in their ecosystem, White-bellied sea eagles control populations of many species of fish, birds, mammals, and reptiles.