The milky stork (Mycteria cinerea) is a type of stork that lives mainly in mangrove forests in Southeast Asia. It is found in Cambodia, Vietnam, Malaysia, and Indonesia. This bird belongs to the genus Mycteria, stands about 36 to 38 inches tall, has a wingspan of 17.1 to 19.7 inches, and a tail length of 5.7 to 6.7 inches. Its feathers are mostly white, except for a few on the wings and tail. Since the 1980s, the number of milky storks worldwide has decreased from 5,000 to 2,000 because of habitat loss, overfishing, and the illegal capture of baby storks. It is listed as Endangered on the IUCN Red List.
Taxonomy and systematics
The milky stork was once classified in the genus Ibis, with the scientific name Ibis cinereus. However, it is now grouped with the genus Mycteria because it looks and behaves similarly to three other storks in this group: the wood stork, yellow-billed stork, and painted stork. Scientific research using DNA analysis and a specific protein has shown that the milky stork belongs to the same group as other Mycteria storks. It is closely related to the painted stork, forming a pair of species that share a common ancestor.
Description
The milky stork is a medium-sized bird that stands 91–97 cm tall, slightly shorter than the painted stork. Its adult feathers are mostly white, except for the black wing and tail feathers, which have a greenish shine. The wings measure 435–500 mm, and the tail measures 145–170 mm. During the breeding season, the white feathers have a pale creamy yellow tint, which is why it is called "milky." This tint disappears outside of the breeding season. The wing coverts and back feathers are lighter and have a nearly white edge.
The bare skin on the face is greyish or dark maroon, with black, irregular spots. During breeding, this skin turns deep wine red with black markings near the bill and throat, and a bright red ring around the eye. After courtship, the face skin becomes a paler orange-red. Breeding birds also have a narrow pinkish band of bare skin on the underside of the wing.
The bill is dull pinkish yellow and sometimes has a white tip. The bill length measures 194–275 mm. The legs are a dull red-flesh color, with tarsi (the lower part of the leg) measuring 188–225 mm. The bird has long, thick toes that help spread its weight on soft mud, preventing it from sinking while foraging.
During courtship, the bill turns deep yellow with a greyish tan on the base, and the legs become deep magenta. Males are slightly larger than females and have longer, thinner bills.
Adult milky storks are easily identified by their white head feathers, yellow-orange bill, and pink legs. They differ from egrets and lesser adjutants by their mostly white body feathers and black wing coverts. However, they may look similar to the Asian Openbill and some white egrets. Egrets are smaller and completely white, while the Asian Openbill has a grey bill. In Vietnam, milky storks sometimes live near painted storks, but painted storks have a black and white breast band, pink inner wing feathers, and more limited bare head skin.
Like other storks, milky storks use warm air currents to travel. Flocks of up to 12 birds may soar high in the air between 10:00 and 14:00. At breeding sites, one bird taking flight often causes others to follow quickly. The average flapping rate during flight is about 205 beats per minute.
When hatching, chicks are covered in white down. Contour feathers (main body feathers) appear by 10–14 days, and full plumage develops after 4–6 weeks. At this stage, the feathers are pale greyish brown with a white lower back, rump, and tail coverts. Some white downy feathers remain under the wings and body, while black wing and tail feathers have a white and dark brown edge. The head feathers are greyish brown, and the bill and skin around the eyes are dull yellow. After 10 weeks, when juveniles leave the nest, the head feathers begin to fall out, revealing dark, bare areas on the forehead and sides of the head. These areas may have dull orange spots. Nestlings have a dark brownish grey bill and skin around the bill and eyes.
By three months old, the head is completely bald, and the bill turns warm yellow with a greenish yellow tip. These features are typical of adult milky storks. Juveniles look similar to painted stork juveniles but can be told apart by the paler underwing lining, which contrasts with the black flight feathers in milky storks, while painted storks have completely black underwing linings.
Milky storks are usually quiet outside of the breeding season. At nests, they make a high-pitched "fizz" sound during the Up-Down display. Young birds croak like frogs when begging for food.
In captivity at zoos such as the National Zoo of Malaysia, Singapore Zoo, and Dusit Zoo, milky storks and painted storks sometimes interbreed, producing hybrid offspring. These hybrids have mixed traits from both species. Because young hybrids look similar to purebred juveniles, scientists use molecular tests to identify them. Adult hybrids have a pink bill instead of an orange one and may have small black spots on the white wings and a faint pink color on their feathers.
At all ages, the milky stork’s eyes are dark brown. The legs are pinkish but often appear white due to the bird’s excreta covering them.
Distribution and habitat
The milky stork lives mainly in Southeast Asia, where it is found in many areas but not all. It is most common in Sumatra, which is its main home, and also lives in Java, Sulawesi, eastern Malaysia, Cambodia, southern Vietnam (where it likely returned after the war from 1963–75), Bali, Sumbawa, Lombok, and Buton. It used to live in southern Thailand but is probably no longer there. A single adult male milky stork found in Setul, Thailand, in 1935 was later discovered in a collection in Singapore, showing that the species once lived in Thailand. This bird was likely the only one recorded there, though some may still visit as a rare guest. The milky stork also visits places like Bali and Sumbawa but lives year-round in Sumatra, Java, and Sulawesi (all in Indonesia). It was first seen in Sulawesi in 1977 when five birds were spotted there. The island of Madura may also have a large group of these birds, as 170 were seen there in 1996.
The milky stork used to live in more areas of Southeast Asia. For example, it once lived along the coasts of the Malaysian Peninsula but now only lives in the Matang Mangrove Forest in Perak.
The milky stork mainly lives in lowland coastal areas, such as mangrove forests, freshwater swamps, peatlands, and estuaries. The only confirmed places where it breeds are in mangroves near its feeding areas. It searches for food on tidal mudflats, in shallow saltwater or freshwater pools, marshes, fishponds, rice fields, and backswamps along river floodplains up to 15 km from the coast. To breed, the milky stork needs large, undisturbed mangrove forests (and possibly forests near rivers or dry areas) with tall trees behind them. It also needs shallow pools in the forest for young birds to feed in. The tall trees are also used for resting, and the birds need enough branches to take off from. When suitable trees are not available, people have suggested using manmade alternatives like wheels attached to poles.
In Peninsular Malaysia, the milky stork is more closely tied to the ocean than the painted stork, which is a related species. However, both species are said to live in the marshy plains of Cambodia, where they likely share the same habitats.
Migration and other movements
The milky stork likely moves short distances during certain seasons outside of when it breeds, but details about when and where these movements happen are not well known. In some areas, milky storks and other wader birds may move when drought begins during the dry season. However, in Cambodia, they leave Tonlé Sap Lake during the wet season, likely heading toward the coast. Reports show milky storks traveling from Sumatra to Java and across the Sunda Straits in September and October. Although these movements are not fully mapped, groups of milky storks are seen traveling more than 200 kilometers in a single day.
Adult milky storks nesting on Pulau Rambut, an island in Java, travel daily to and from the island to feed in fishponds and rice fields on the mainland of Java. The breeding group on Pulau Rambut may also be visited by different numbers of migrating birds throughout the year.
Behaviour and ecology
Breeding usually happens after the rainy season, which can last from April to November. The start of breeding can happen at different times in different areas, but it usually lasts about three months and likely happens when fish are most plentiful after the rainy season. In South Sumatra, for example, the milky stork breeds from June to September, and has been seen with breeding feathers as early as February. In Java, eggs have been found as early as March, with one report of baby storks leaving the nest in July (Hoogerwerf, 1936). Breeding in Java is most common from July to August. At Tonle Sap in Cambodia, egg-laying can begin as early as January or February, since the dry season starts even earlier there. In Malaysia, eggs have only been found in nests in August.
The milky stork builds nests in groups in mangrove swamps. These groups can range in size from 10–20 nests to a few hundred nests. In Java, groups with 75–100 nests have been estimated to cover 4.5 hectares, with 5–8 nests in each tree. The height of nests above the ground varies. Nests are often built in tall Avicennia marina trees and other Avicennia species in Java, and Rhizophora apiculata trees in Sumatra, usually 6–14 meters above the ground, but sometimes in the very tops of trees. On Pulau Rambut, nests are built in the tops of very tall mangrove trees that are 30 meters high. Similar nesting habits were seen on Pulau Dua when the species still bred there. In Indonesia, some milky storks nest close to the ground in dense shrubs of Acrostichum mangrove ferns that are 2–6 meters high. Milky storks also nest in dead or dying mangrove trees. In South Sumatra, some colonies are found far inland at brackish lakes or freshwater swamps in Alstonia trees that can reach 60 meters high.
The nests are strong, large structures about 50 centimeters in diameter, made mostly of medium-sized sticks from Avicennia trees, with many leaves still attached. These nests look similar to those of the grey heron and the white ibis but are slightly stronger and made of thicker twigs. Other nests found are small and weak, like those of doves. When collecting nesting material, milky storks break off live branches by grabbing the branch with their beak and flying upward a short distance, which can be hard and sometimes fails. If the branch does not come free, the stork moves to another one. Nest building continues even when young birds are in the nest.
Clutches usually have 1–4 eggs, but 2–3 eggs are most common. The eggs are about 59.0–74.5 millimeters long and 43.0–48.0 millimeters wide. They are small compared to the adult body size and look similar to those of Leptoptilos javanicus but are slightly lighter in color. The incubation period lasts about 27–30 days. The first and last eggs may hatch several days apart, so the oldest and youngest chicks can be very different in size. Both parents take turns sitting on the eggs. When parents switch nest duties, the returning parent and the parent sitting on the eggs greet each other with loud, quick beak clattering, along with deep head bowing and neck stretching. When disturbed, the brooding bird performs an arching display similar to other Mycteria species.
Courtship involves both partners bowing and raising their bills repeatedly while standing opposite each other and performing the display in a mirror-like way. Many displays at the nest look like those of other Mycteria species. When a male sees a female arriving at the nest, he uses display preening to attract her, and she responds with a balancing posture and gaping. An up-down greeting display follows the arrival of one partner, and the male performs a flying-around display when the female arrives. Both partners at the nest pull back the skin on their heads to expose more bare skin than usual during displays.
In South Sumatra, the milky stork breeds with lesser adjutants, black-headed ibis, and various heron species. In Cambodia, milky storks have been seen breeding with painted storks, lesser adjutants, and spot-billed pelicans in flooded forests near Tonle Sap during the early dry season in January and February. These colonies are in mangrove backswamps 1–4 kilometers from the coast, in dense Archostichum ferns or dead trees.
Breeding in Malaysia is likely rare and not very successful. In 1984, adult storks were seen with breeding feathers in July at Kuala Gula, and about 20 nests were reported there in 1989, along with an increasing adult population. Before these observations, no breeding signs had been recorded since 1935. However, no baby storks have been seen in Malaysia since 1983, and the lack of successful breeding is probably due to high predator pressure. Breeding no longer happens on the Javan island of Pulau Dua. Since the mid-1970s, this island has been connected to the Javan mainland through coastal changes, leading to deforestation for firewood. Breeding stopped because tall trees, needed for nesting, were removed. However, some storks are still seen on the island during courtship, and some suggest breeding could return with strong conservation efforts. Some breeding still happens on Pulau Rambut; a few breeding pairs were observed there in 2014 near Jakarta Bay.
Young captive milky storks can become sexually mature at three months old, but they often act too early and are not experienced, so actual breeding likely happens a bit later. In the wild, young storks leave their birthplace at 3–4 months old. In captivity, milky storks sometimes breed with painted storks. A hybrid between a male lesser adjutant and a female milky stork was born at Jurong Bird Park, Singapore.
The milky stork eats a variety of food. In Malaysia, its main diet includes mudskippers from the Periophthalmus and Gobiidae families that are 10–23 centimeters long. In South Sumatra, catfish (Arius spp.) may be a main part of the diet. Other prey in South Sumatra includes milkfish, giant mudskippers (Periophthalmodon schlosseri), mullet species (Moolgards and Chelon), eel catfish (Plotosus canius), fourfinger threadfin (Eleutheronema tetradactylum), and Chinese silver pomfret (Pampus chinesis). Snakes and frogs are also eaten, especially to feed young. Parents give nestlings large fish, eels, and mudskippers
Threats and survival
The number of milky storks around the world has been decreasing a lot, especially since the late 1980s. This decline is mainly because of habitat loss and disturbance caused by cutting down mangrove forests for human activities like fish farming, rice cultivation, logging, and in Indonesia, moving people into new areas. Cutting down trees has made it harder for milky storks to find safe places to nest, which has hurt their ability to breed successfully. Breeding groups have also decreased due to illegal trade of the birds, especially in South Sumatra, where people have stolen eggs and young birds to eat. Young milky storks have also been sold to zoos in Singapore, Kuala Lumpur, Brunei, and several European zoos. The milky stork is vulnerable to human activity, which may explain why the species has declined so much. While fishponds and shrimp farms can sometimes provide extra food sources, these manmade structures can also disturb nearby nesting areas and harm the population over time.
The milky stork was almost completely removed from Vietnam because of the destruction of mangrove swamps during the Southeast Asian War from 1963 to 1975. Later, large-scale replanting of trees may have allowed some storks to return, but the species has also suffered long-term effects from the use of chemicals in Southern Vietnam.
Natural predators of the milky stork include monitor lizards, such as Varanus salvator, which eat eggs and young storks in South Sumatra. Crocodiles also sometimes eat young storks. In Matang Mangrove Forest in Malaysia, brahminy kites, water monitor lizards, and common palm civets are potential predators that may harm the survival of the species. In captivity in Malaysia, crab-eating macaques, which can swim to nest sites, may also threaten stork eggs and chicks. On Pulau Rambut, reticulated pythons, cat snakes, Brahminy kites, and white-bellied sea eagles are reported to prey on stork chicks.
Another threat is pollution in the milky stork’s habitat from high levels of metals like copper, zinc, and lead. These metals come from the use of chemicals in Kuala Gula, corrosion and runoff from jetties and boats coated with these metals, and aquaculture development. On Pulau Rambut, breeding colonies may also be harmed by increasing ocean pollution.
Efforts to save the milky stork have focused on breeding them in captivity. In 1987, the first captive breeding and reintroduction program for the milky stork began at Zoo Negara in Malaysia, where the wild population had dropped by over 90% from the 1980s to 2005. This program started with five male and five female young storks from Singapore Zoo and Johor Zoo. By 2005, the captive population had grown to over 100 individuals. As the wild population declined, the number of captive, free-flying storks has increased steadily.
The first successful reintroduction of milky storks into the wild happened in 1998 at Kuala Selangor Nature Park, where 10 captive-bred storks were released. Matang Mangrove Forest in Malaysia has been chosen as a reintroduction site because it has suitable shelter, breeding, and feeding areas, and there are few threats from predators or parasites. The area is protected and managed carefully, and natural resources are used sustainably. However, released storks may face new dangers, such as being more likely to get sick, struggling to find food or defend their territory, and not knowing how to avoid predators. These animals may also carry diseases that could harm wild storks.
Captive storks also face the risk of losing genetic purity if they mate with painted storks, which can happen in captivity or in the wild where the two species overlap in northern parts of the milky stork’s range.
Conservation of the milky stork in the wild has depended on creating and protecting wetland areas, as seen in Vietnam’s reforestation and protection efforts, which may have helped the species return to some areas. Raising public awareness about the milky stork’s endangered status is also a key step in its conservation.
In culture and relationship to humans
The milky stork is well known to people in Indonesia and can be easily recognized in the wild compared to other wading birds. It has been hunted for its meat and eggs, sometimes illegally, in the past. Like many wading birds, it is sometimes seen as a minor problem in aquaculture because it eats fish and shrimp. However, the milky stork is officially protected in Malaysia and Indonesia. It has been listed on Appendix I of CITES since 1987, which means it is strictly protected internationally.
This stork has been kept in zoos such as Vienna Zoo and Washington Park Zoo. The longest-living milky stork in captivity lived 12 years, 4 months, and 18 days (1937–1950) at Washington Park Zoo. Some young storks in zoos have been mistakenly identified as painted storks when purchased, but later proven to be milky storks. The only zoos recorded as successfully breeding this species are Zoo Negara (where the first captive breeding occurred in 1987), San Diego Zoo, and Singapore Zoo. Government and non-government groups, such as Perhilitan, the Malaysian Nature Society, and the Malaysian Zoological Society, have supported captive breeding and reintroduction programs for this bird.
The image of a flying milky stork appears on the logo of Sembilang National Park. It is also frequently used on calendars and posters in public awareness campaigns in Sumatra.
Status
In 2023, the global population of milky storks was estimated to be between 1,200 and 1,850 adult individuals, a decrease from about 5,000 in the 1980s. In Malaysia, the number of milky storks dropped steadily from over 100 in 1984 to fewer than 10 by 2005, a decline of more than 90%, putting the local population at risk of extinction. Most of the current global population lives in Indonesia. There are likely around 800 to 1,600 individuals in Sumatra, about 100 to 200 in combined populations on Java and Sulawesi, and very few or none remaining in breeding populations on the Southeast Asian mainland. The population in Cambodia is very small, with about 100 to 150 individuals; although this group may be relatively stable, rapid declines could occur if threats continue. Because of large population declines across its range, the milky stork's status was changed to Endangered from Vulnerable in 2013 by the IUCN.