The White-Bellied Sea Eagle

Introduction

Hong Kong is one of the most densely urbanized regions in the world. Yet, the white-bellied sea eagle, a large raptor species notorious for being negatively affected by human disturbance, thrives in Hong Kong more than in any other region of the world.

Although you are very unlikely to ever see the white-bellied sea eagle soaring over the skyscrapers of Central or Kowloon, the population of white-bellied sea eagles remains remarkably stable in the country parks and special areas of Hong Kong, such as the outlying islands or Sai Kung. 

First observed in Hong Kong at the tail end of the nineteenth century, the population of white-bellied sea eagles in the territory has paradoxically increased as the city grew, even as populations in other parts of the world where the species was known to live declined.

Description

The white-bellied sea eagle was first described as the white-bellied eagle in 1781 by English naturalist John Latham, based on a specimen that had been brought back to England in February 1780 from Prince’s Island, off the westernmost coast of Java, in Indonesia, during Captain James Cook’s last voyage. Seven years later, in 1788, German naturalist Johann Friedrich Gmelin formally described the specimen as Falco leucogaster; “leuko” being the Ancient Greek term for “white,” and “gaster” for “belly.” Hence, its original English name, the white-bellied eagle.

However, in 1843, French naturalist Marie Jules César Savigny introduced a new taxonomy and placed this species of eagle in the genus Ichthyophaga, which, including the white-bellied sea eagle, comprises ten distinct species (Tsim et al., 2003). The term “ichthyo” literally means “fish,” and “phaga” means “eater.” In other words, Ichthyophaga leucogaster literally means “white-bellied fish eater,” as the white-bellied sea eagle mainly preys on marine animals. However, it is also commonly referred to as Haliaeetus leucogaster; “hali” and “aetos” being the Ancient Greek words for “sea” and “eagle,” respectively.

The white-bellied sea eagle is a monotypic species (Tsim et al., 2003; So & Lee, 2010), meaning that there are no subspecies of white-bellied sea eagles. However, along its closest relative, Sanford’s sea eagle (Ichthyophaga sanfordi or Haliaeetus sanfordi), also known as the Solomon eagle—a species endemic to the Solomon Islands—the white-bellied sea eagle forms a superspecies. The primary difference between both species lies in the color of their heads: the white-bellied sea eagle has a white head, while Sanford’s sea eagle has a dark head.

Other than its head, the white-bellied sea eagle displays a white breast and abdomen (Clunie, 1994), while its back, tail, and the upper side of its wings are grey. The underside of its wings is predominantly grey, with a white area around the leading edge (Marchant & Higgins, 1993; TGDNRE, n.d.), which gives this species a distinctive white/dark pattern when seen in flight from below. Its hooked bill is bluish grey with a dark tip, and its unfeathered legs are either yellow or grey, with dark talons.

The white-bellied sea eagle is a large bird of prey, but there is little to no dimorphism between genders. Males, standing 60–80 centimetres (23.5–31.5 inches) tall and weighing 1.8–3.0 kilograms (4–6.5 pounds), are slightly smaller than females (Tsim et al., 2003), which are 80–90 centimetres (31.5–35.5 inches) tall and weigh 2.5–4.5 kilograms (5.5–10 pounds). Their wingspan ranges from 180 to 220 centimetres (5.8 to 7.2 feet) (TGDNRE, n.d.).

Young white-bellied sea eagles, however, differ vastly from their parents. In their first year, young individuals are mostly brown, with a lighter shade on their head, neck, and abdomen. Incidentally, when naturalists first discovered Sanford’s sea eagle, they misidentified that species as young white-bellied sea eagles due to their very similar coloration.

As young white-bellied sea eagles grow, the plumage on their head, neck, and abdomen gradually lightens until it becomes entirely white once they reach adulthood. This transition occurs through a series of moults over the span of four to five years (Marchant & Higgins, 1993). They reach sexual maturity at the age of five (Debus, 2008) and have an estimated average lifespan of 20 years in the wild.

Distribution & Habitat

The habitat of the white-bellied sea eagle spans a wide area, extending from the shorelines of southern China, including Hong Kong, across the coasts bordering the South China Sea (southern China, Vietnam, the Philippines, and Malaysia), the eastern Arabian Sea (western coastline of India), the Bay of Bengal (eastern coastline of India, Sri Lanka, Bangladesh, Myanmar, and Thailand), the Celebes Sea (southern Philippines and northern Indonesia), the Java Sea (central Indonesia), the Banda Sea (eastern Indonesia), and along the entire coastlines of Papua New Guinea and Australia, including the island of Tasmania (Mayr & Cottrell, 1979; Marchant & Higgins, 1993; Clunie et al., 1994; del Hoyo et al., 1994; Tsim et al., 2003; So & Lee, 2010).

Although white-bellied sea eagles are primarily active along coastal areas such as inshore seas, offshore islands, coasts, estuaries, and wetlands (Robson, 2000; Chow, 2003; Tsim et al., 2003), they are also known to travel vast distances inland, following rivers and other water bodies while hunting for fish, turtles, bats, and other prey animals.

Unlike other birds of prey that are commonly seen flying in central and urbanized regions of Hong Kong, white-bellied sea eagles are easily disturbed by human activity, especially during the breeding season. Consequently, they are most frequently observed flying around the outlying islands and less urbanized areas of Hong Kong (So & Lee, 2010).

Behavior

Adult white-bellied sea eagles tend to form pairs, and are sedentary and territorial (Clunie, 1994), while young and immature white-bellied sea eagles are more prone to be nomadic (So & Lee, 2010) and are often found at vast distances from their birthplace. They are also monogamous; pairs stay together until one of them dies, after which the surviving bird finds a new mate (Clunie, 1994; Tsim et al., 2003; So & Lee, 2010). However, on one occasion in Hong Kong, a paired female was observed copulating with an intruding male (Tsim et al., 2003).

The breeding habits of the white-bellied sea eagle are poorly known. Depending on the location, the breeding season may vary (Bilney & Emison, 1983). For instance, in Australia (southern hemisphere), breeding occurs between June and August; whereas in Hong Kong (northern hemisphere), it takes place between December and May (So & Lee, 2010). Before mating, pairs of white-bellied sea eagles can be seen courting each other, involving behaviors such as diving, gliding, and chasing, all accompanied by loud calls.

Breeding typically occurs in tall trees with good visibility, overlooking a body of water, on ledges above cliffs, or on the ground (Clunie, 1994; Chow, 2003; So & Lee, 2010), far from potential predators that prey on eggs. On rare occasions, nests are also built on artificial (i.e. man-made) structures (Clunie, 1994; So & Lee, 2010).

The nest-building process, which results in a structure measuring 120–170 centimeters (3.9–5.6 feet) wide and 50–180 centimeters (1.6–5.9 feet) deep (Debus, 1998; Tsim et al., 2003; So & Lee, 2010), can take up to six weeks before the female lays her eggs. Since white-bellied sea eagles are territorial, they reuse the same nest year after year (del Hoyo et al., 1994), and the nest grows larger with each passing year as it requires renovation. The nest is constructed of sticks and branches, with a lining made of grass and seaweeds (Clunie, 1994; Debus, 1998).

A female white-bellied sea eagle usually lays two whitish-yellow, oval eggs measuring approximately 7 centimeters (2.8 inches) in length and 5.5 centimeters (2.2 inches) in width, which are incubated for about six weeks by both parents alternately (Tsim et al., 2003). When hatching, the chicks are covered in white down. Although the female lays two eggs, typically only one chick is successfully raised to fledging; the other egg may be infertile or the weaker chick may die in the nest. Chicks reach fledging between 70 and 80 days of age, but they usually spend around six months in their parents' territory.

Feeding

The white-bellied sea eagle relies heavily on the marine ecosystem as a food source. The majority of its diet consists of marine animals, including fish, turtles, shellfish, sea snakes, and others. However, it also preys on small land animals such as rabbits, marsupials, small reptiles, and other birds, including penguins in Western and southern Australia. Additionally, it is an opportunistic feeder and readily consumes carrion (Clunie, 1994; Tsim et al., 2003; Debus, 2008).

When hunting over water, the white-bellied sea eagle flies low over the surface, holding its talons forward and poised to strike unsuspecting fish (Clunie, 1994). It has also been observed diving from a perch overlooking a lake, remaining submerged for a short period to catch a fish (Debus, 1998 & 2008; Tsim et al., 2003). Pairs have also been noted hunting together (Clunie, 1994).

When they fail to catch prey themselves or when the opportunity arises, white-bellied sea eagles have been seen harassing other predatory birds, including smaller birds of prey, seagulls, cormorants, gannets, and even seals, in an attempt to steal their catch (Clunie, 1994; Tsim et al., 2003). On some occasions, white-bellied sea eagles have also been observed stealing food from their own species.

Hong Kong

The white-bellied sea eagle was first reported in Hong Kong between 1887 and 1907, after a specimen was collected and shipped to the British Museum of Natural History in London. It is part of the Styan Collection, named after Frederick William Styan, an Englishman who worked as a clerk for the tea-trading company Anderson and Co. in Shanghai and Foochow (present-day Fuzhou) between 1895 and 1903. He is, however, best remembered for his contributions to ornithology and had several bird species (and a rare Chinese snake species) named after him.

The white-bellied sea eagle is one of twenty-seven raptor species observed in Hong Kong (So & Lee, 2010), but it is only one of the nine raptor species that breed in Hong Kong (Carey et al., 2001; Tsim et al., 2003). They are mostly seen along coastlines, offshore islands (So & Lee, 2010), and sometimes around reservoirs (Carey et al., 2001). Although white-bellied sea eagles mainly fly singly or in pairs, groups of five have been observed in Hong Kong (So & Lee, 2010).

In Hong Kong, as observed elsewhere, white-bellied sea eagles tend to avoid urbanized regions. A study conducted in 2008/2009 observed fourteen pairs of white-bellied sea eagles in natural environments; only one pair was seen on a cable tower in Aberdeen (So & Lee, 2010). The vast majority of pairs nested either on offshore islands or in the less densely inhabited areas of Sai Kung. Not a single pair was observed on Hong Kong Island or in the highly urbanized Kowloon; though individual birds have been seen flying in the southern part of Hong Kong Island.

White-bellied sea eagles also seem to favor eastern and southern Hong Kong rather than western Hong Kong (Tsim et al., 2003), with fewer individuals and/or nests reported in the northwestern New Territories or western Lantau Island. This may be due to the dominant easterly winds during the breeding season, making the western part of Hong Kong less suitable for nesting.

One notable case in Hong Kong is that of Hailey: an injured male juvenile white-bellied sea eagle rescued by the Society for the Prevention of Cruelty to Animals (SPCA) and given to the Kadoorie Farm & Botanic Garden on May 29, 1997. Hailey was only 2 to 3 years old when he was attacked by black kites, a common raptor species in Hong Kong. After four years of care, the Rescue Team attempted to release Hailey, but he encountered trouble again and was rescued once more. He ultimately spent 17 years at Kadoorie Farm & Botanic Garden, where he became an Ambassador Animal. Hailey passed away on May 5, 2014, at the age of 19 (KFGB, 2014).

Conservation

Globally, the white-bellied sea eagle is classified as “Least Concern” on the IUCN Red List of Threatened Species, although the population is decreasing, with the number of mature individuals estimated to be between 2,600 and 41,000 (IUCN Red List, 2026). However, populations of white-bellied sea eagles vary between locations. In Thailand, mainland Southeast Asia, and southern Australia, numbers have declined (Zheng & Wang, 1998; MacKinnon & Phillipps, 2000); in China, the white-bellied sea eagle is categorized as being in National Protection Class II, the second-highest tier of protected wild species under the Wildlife Protection Law of the People's Republic of China (NPC, 2007).

Habitat destruction and human disturbance appear to be the main threats to the white-bellied sea eagle, primarily due to the loss of nesting sites (Clunie, 1994; So & Lee, 2010). Adults have been observed abandoning their young following human disturbance (Bilney & Emison, 1983). Conversely, artificial infrastructure such as reservoirs and dams may have contributed to an increase in inland white-bellied sea eagles, as stabilized water levels and the resulting increase in food supply provide favorable conditions (Bilney & Emison, 1983; Clunie, 1994).

In Hong Kong, the white-bellied sea eagle is considered an uncommon resident (Tsim et al., 2003). Like all wild birds, including their nests and eggs, it is protected under the Wild Animals Protection Ordinance (Cap. 170) (Tsim et al., 2003; Hong Kong e-Legislation, 2022; AFCD, 2025).

However, contrary to many other locations, the number of white-bellied sea eagles in Hong Kong is increasing. In the 1930s and 1940s, only three breeding pairs were observed annually (Carey et al., 2001; So & Lee, 2010). By 2003, that number had increased to five pairs of adults and eleven solitary individuals—a total of twenty-one individuals (Chow, 2003). Other research reported that the number was thirty-nine identified individuals (Tsim et al., 2003). In 2009, the population had further grown to fifteen adult pairs, with an estimated twenty-six immatures and juveniles, as well as a one-year-old fledgling, bringing the total to fifty-seven. However, this number could be lower due to juvenile mortality (So & Lee, 2010).

Although fifty-seven may seem small, the territorial behavior of the white-bellied sea eagle and nesting patterns observed in Australia (cf. Ferguson-Lees & Christie, 2001) suggest that the coastline of Hong Kong has reached its maximum capacity for this species (Tsim et al., 2003).

Conclusion

Hong Kong’s population of white-bellied sea eagles has been thriving over the past decades. Although their breeding habits remain poorly understood—partly because they build nests in trees and on cliffs that are not easily accessible—the population of breeding white-bellied sea eagles has successfully produced offspring over the years.  

Unfortunately, due to the high natural mortality rate observed in juvenile white-bellied sea eagles, their population does not increase as quickly as one might expect. However, as studies and research conducted in Australia, Tasmania, Thailand, or China have shown, it is very likely that some of the juveniles born in Hong Kong have dispersed outside the territory, as the population density may have reached its maximum in Hong Kong.  

Although populations of white-bellied sea eagles are declining in other parts of the world where the species is commonly found, Hong Kong remains one of the very few regions where the white-bellied sea eagle continues to thrive.

Reference List

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