The White-bellied Sea Eagles of northern Penang Island, Malaysia

Introduction

The White-bellied Sea Eagle is a large diurnal bird of prey that breeds and hunts near water, with fish forming the majority of its diet. The adult 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. Like many raptors, the female is slightly larger than the male. Immature birds have brown plumage, which is gradually replaced by white over 5-6 years until the adult plumage is obtained.The lifespan is 20 years and the adults weigh 1.8-4.5kg and are 66-90cm in length. The wingspan is 1.9 to 2.2 metres. The mature juveniles are a similar size to the adults.

Adult

Juvenile

Mature juveniles from below

Juvenile from above

Adult from below

Adult with a fish

Juvenile from below

Distribution

White-bellied Sea Eagles are found from Mumbai eastwards in India, Bangladesh and Sri Lanka in Southern Asia, through all of coastal Southeast Asia including, Myanmar, Thailand, Malaysia, Indonesia and Indochina, the main offshore islands of the Philippines and Southern China including Hong Kong, Hainan, and Fuzhou, eastwards through New Guinea and the Bismarck Archipelago, and Australia. These eagles occur mainly in coastal areas, islands, estuaries but also in large inland water bodies, lakes, rivers and wetlands.

Local Observations

My residence is on an upper floor of Diamond Villa which is positioned in Tanjung Bungah on the Southern shores of the Strait of Malacca. The estate is backed by the forest clad Pearl Hill where several pairs of adult eagles have their nests. Members of the eagle families catch fish in the Strait and circle above the forest and in close proximity, at times, to our condominium windows, to gain altitude to fly to their nesting or perching trees.

Location of Tanjung Bungah, near George Town, Pinang

Tanjung Bungah and Pearl Hill. Yellow arrow = eagle's nest. red arrow = my residence, yellow line = path eagles take from the Strait to their nest.  

Strait of Malacca and Mt Jerai on the mainland

Incoming eagle flying close to Strait-side condominiums.

The forest on Pearl Hill

The Strait of Malacca is the busiest waterway in the world.

Habits and Lifestyle

White-bellied sea eagles are generally territorial; some birds form permanent pairs that inhabit territories throughout the year, while others are nomadic. Immature birds are generally dispersive, with many moving over 50km away from the area where they were raised. They  are diurnal and often seen perched high in a tree or soaring over waterways or adjacent land. A pair may cooperate to hunt. During hunting the bird prepares for the strike by holding its feet far forward and then strikes backwards while simultaneously beating its wing to lift upwards. They often catch a fish flying low over the water and grasping it in one talon.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 even when perched.

Individuals often have a brief mid-flight squirmish with another of their species. They are often chased or harassed by local House Crows. There are other raptors in the area, mostly the smaller Brahminy Kites

Juvenile preening

Juvenile being chased by a House Crow

Juvenile being mobbed by House Crows

Rival in the skies; a Brahminy Kite

Diet

White-bellied sea eagles are opportunistic carnivores and consume a wide variety of animal prey, including carrion. They hunt mainly aquatic animals,  as fish, turtles and sea snakes, but they also take birds and small mammals. Images have appeared showing these eagles carrying wild piglets and penguins.

Locally, a variety of fish 'fly' past our windows. Of note are Puffer fish (Fugu in Japan) which contain a powerful toxin in some of their internal organs. Highly trained specialist chefs are required to prepare the fish , which is regarded as a delicacy in Japan. Quite how the sea eagles avoid the tetrodotoxin is not known. The toxin, a sodium channel blocker,  is in the liver, ovaries, eyes and skin.

With a puffer fish

Another puffer fish

Nesting and Breeding

White-bellied Sea Eagles invest an immense amount of energy into breeding; raising young occupies much of their activities for six to seven months of the year.

This species is monogamous. The local pair have a large permanent nest tucked into the forest around Pearl Hill ridge. Around late November they can be seen flying with large branches to enhance the nest. The egg-laying platform is lined with vegetation and leaves and constantly added to. Around the end of the year the adult pair perform skillful displays of flying such as diving, gliding and chasing each other while calling loudly. They mirror each other, flying 2-3m apart and copy 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 talon with its own. 

Carrying soft vegetation to line the egg cavity.

Mating occurs near the nest and after which 1 or 2 eggs are laid. The incubation period is six weeks. During this time the two sexes will take turns sitting on the nest while the other one hunts. Changeovers are spectacular.

When the eaglet(s) emerge they are covered in white down. For most of this next 6 months the female tends the chicks while the male hunts or stands guard nearby or flies above the nest.

He will dive on intruders such as the image below (multiple exposure)

The chicks are fed with a constant stream of food and will be close to the size of the parents when they fledge.

The progress to fledging is summarised in the following composite image.

Locally, I have observed nesting for 3 seasons. The return is not very good with nesting aborted on two of these years. One year it appeared that a storm displaced some of the finer branches and lining at the top of the nest. The nest is very exposed both to intense sunlight and from high winds and torrential rain. in 2021 the nest came under attack from a juvenile, which was attacked by the male near the nest and met a very stout defense by the female at the nest. A week after this attack the nest was abandoned for that year.

Population Number

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 breeding pairs. Currently this species is classified at Least Concern (LC) on the IUCN Red List BUT its numbers are decreasing. 

The nesting failures I have observed do not seem due to human interference but preservation of forests and suitable nesting trees is essential for their long-term survival.