Anastomus oscitansAsian openbill

Geographic Range

Asian openbills are native to the oriental biogeographic region and are largely found in India, Sri Lanka, and certain parts of Indochina. They occupy their entire native range all year round, migrating to specific destinations during breeding season. (Bock, 2002; Manna, 2009)


Asian openbills inhabit wetland habitats including flooded fields, shallow estuarine marshes, and lakes with brackish water. Flooded fields are often agricultural areas and serve as rice paddies. Such wetlands are on average 385 to 1100 m above sea level and 10 to 50 cm in depth. Asian openbills are wading birds and as such, require ample amounts of rainfall for moist feeding grounds. Nests are built on tree branches which are usually 15 to 60 ft above the ground. (Bock, 2002; Jayson, 2002; Jha, 2012; Manna, 2009; Nayak, 2012)

  • Range elevation
    385 to 1100 m
    1263.12 to 3608.92 ft
  • Range depth
    0.1 to 0.5 m
    0.33 to 1.64 ft

Physical Description

Asian openbills are medium-sized storks. They are, on average, 81 cm long with a wingspan ranging from 147 to 149 cm. Their mass has not been well-documented, however, storks typically weigh anywhere from 1.3 to 8.9 kg. Asian openbills have pale white or gray plumage with black wings and a forked black tail. Their legs are red and their bills are a dull, yellow-gray color. A notable feature is their open bill, formed by the downward curvature of their lower mandible, which only meets the upper mandible at the tip. Asian openbills are often mistaken for herons, as are other stork species. Storks generally are of a heavier build and fly with their necks outstretched as opposed to retracted. (Bock, 2002)

Males and females are sexually monomorphic, and are usually distinguished by position during copulation rather than by physical appearance. The young contain traces of brown in their plumage, enabling them to be easily distinguished from adults. (Manna, 2009)

  • Range mass
    1.3 to 8.9 kg
    2.86 to 19.60 lb
  • Average length
    81 cm
    31.89 in
  • Range wingspan
    147 to 149 cm
    57.87 to 58.66 in


Asian openbills are largely monogamous, however rare cases of polygyny have also been reported. Monogamous pairs typically occupy nesting sites in trees early on during breeding season. Male storks, which arrive at these trees after they are fully occupied are compelled to share nesting sites, hence engage in intraspecific attack. In doing so, they attract several single male storks to join the fight. In the end, a male stork joins the monogamous pair or replaces the male which the female originally mated with. Males in polygynous pairs exhibit displays in the same manner as do those of monogamous pairs. Members of polygynous nests generally share the responsibilities of nest-building, incubation, and caring for offspring in a non-discriminant manner. Polygynous nests are also highly successful in terms of hatching, fledging, and protection against intraspecific attack. (Datta, 1995)

Courtship involves male display of nest-building behavior, a sexually-selected trait. Males do this by showing females potential nesting sites and manipulating materials for nest construction. In doing so, males demonstrate the qualities of their genes and willingness to invest in reproduction. Females choose good nest-builders so they can save energy and maintain good physique to meet the costs of reproduction. (Manna, 2009)

During the mating process, Asian openbills fly near each other, often above one another, and eventually pair. Males and females rest side by side on a branch for hours, pecking each other’s heads and behaving aggressively. During copulation males stand on females backs and touch females necks with their bills while establishing cloacal contact for sperm transfer. (Manna, 2009)

Asian openbills breed on an annual basis, usually between June and December. Breeding reaches its peak during the monsoon months when there is sufficient rainfall to moisten feeding grounds. Both parents work together to build the nest using leaves, grasses, branches, and twigs. Females lay 2 to 5 eggs per reproductive cycle. Both parents incubate the eggs until they hatch at 27 to 30 days. The nestlings are completely dependent on the parents through fledging at 35 to 36 days and continue to remain dependent until reaching sexual maturity at 60 days. At this time, the nestlings leave the nest and are able to breed within the same breeding season in which they have hatched. (Bock, 2002; Manna, 2009)

  • Breeding interval
    Breeding season for Asian openbills occurs once each year.
  • Breeding season
    Asian openbills breed from June to December.
  • Range eggs per season
    2 to 5
  • Average eggs per season
  • Range time to hatching
    27 to 30 days
  • Range fledging age
    35 to 36 days
  • Average time to independence
    60 days
  • Average age at sexual or reproductive maturity (female)
    2 months
  • Average age at sexual or reproductive maturity (male)
    2 months

Both parents make significant investments throughout the various reproductive stages in a cooperative fashion. During nest-building, males generally gather the construction materials while females defend the nesting site. Both sexes construct the nest together, with the female typically taking charge. After the female lays the eggs, both sexes take turns incubating them while simultaneously enlarging the nest. During hatching, both parents work together to drop the shell and other particles. When the offspring are very young, both parents are continuously present at the nest to protect them from predators and harsh weather conditions. When the offspring become more mature, the parents take turns foraging, however the females spend more time in the nest. Both parents partake in feeding and watering their offspring. (Manna, 2009)

  • Parental Investment
  • altricial
  • male parental care
  • female parental care
  • pre-hatching/birth
    • protecting
      • female
  • pre-weaning/fledging
    • provisioning
      • male
      • female
    • protecting
      • male
      • female
  • pre-independence
    • provisioning
      • male
      • female
    • protecting
      • male
      • female


The longest lifespan of Asian openbills in captivity is 18 years. However, in one study, an Asian openbill stork was found to survive for more than 18 years in captivity. (Brouwer, 1992)

  • Range lifespan
    Status: captivity
    18 (high) years


Asian openbills are diurnal and migratory. They fly by soaring on thermals then glide to their destination. They are highly social and form large nesting colonies in trees with other storks and waterbirds such as herons. Nests are stratified to facilitate division of resources among the co-existing species and accommodate for interspecific variations in nest size. Asian openbills have relatively tall nests, and as a result, exclusively occupy the topmost story of the tree. Colonial nesting is strategic in that large groups of storks effectively defend the colony against predators. Such territorial behavior is also seen among mating pairs within the same species. Members of a pair often defend their nests from intraspecific attack. (Bock, 2002; Datta, 1995; Jha, 2012)

Home Range

Although the exact home range of the Asian openbill stork is unknown, field studies have provided estimations using various parameters. In one study, the average colony was found to contain 150 nests, each of which was roughly 100 cm in length and had a radius of 30 cm. The storks remained in close proximity to their colonies, venturing as little as 1 to 1.5 km from them to obtain resources. In actuality, this species home range is much larger when taking into account its migratory range during breeding season. (Manna, 2009)

Communication and Perception

Asian openbills rely heavily on sight and touch to perceive their environment; however studies involving closely related species such as turkey vultures suggest that they may also utilize olfactory cues. Enlarged olfactory bulbs make olfaction possible. Asian openbills, like other storks, are largely mute due to the absence of syrinx muscles, hence vocalization is minimal. Their calls can be described as a mournful “hoo-hoo” and at very close distances, they can be heard making a low grunting noise. Asian openbills resort to bill-clattering as their primary method for various forms communication. Bill-clattering also serves as an important form of communication during breeding season. ("Stork", 2011; Bock, 2002; Pough, 2009; Stuart, 1929)

Food Habits

Asian openbills are carnivores. Their diet primarily consists of golden apple snails and small aquatic invertebrates such as molluscs, crabs, and worms. Frogs, lizards and snakes, insects, and fish also make up a large part of their diet. Asian openbills leave their colonies from time to time in search of food. They typically feed alone, but may form flocks in areas saturated with food. Asian openbills wade in the water and locate their prey using touch and sight. Sometimes, they are seen stalking their prey in an effort to capture them. In most cases, these storks swallow their prey whole, however, they may use their sharp, pointed lower mandibles to crush hard-shelled prey and extract their flesh. ("Stork", 2006; Bock, 2002; Stuart, 1929)

  • Animal Foods
  • amphibians
  • reptiles
  • fish
  • insects
  • mollusks
  • aquatic or marine worms
  • aquatic crustaceans


The eggs and nestlings of Asian openbills, are commonly preyed upon by crows, Indian spotted eagles, and monitor lizards. This usually occurs at times when parents are away from the nest during incubation and chick-rearing periods. Asian openbills employ several anti-predator adaptations. Parents take turns incubating and foraging to more effectively defend their offspring from predators. Asian openbills also form mixed colonies with other stork species and waterbirds such as herons to establish safety in numbers. Colonies are isolated from their surroundings in a moat-like fashion and nests are typically built in tall trees to make them more difficult for predators to reach. (Bock, 2002; Jha, 2012; Manna, 2009)

Ecosystem Roles

Asian openbills serve as effective indicators of the ecological health of wetlands. They are also vital components of wetland ecosystems in that they establish significant links in food webs and nutrient cycles. Asian openbills produce fecal matter that is rich in nitrogen and phosphorus, serving as an effective form of fertilizer for wetland plants. This in turn causes substantial increases in fish and crab populations which feed on them. Asian openbills feed on golden apple snails , a major rice pest in Asia, and effectively reduce their populations. (Bhartari, 2008; Sin, 2003)

Trematodes (Chaunocephalus ferox) commonly use Asian openbills as a host species. They inhabit the small intestine, producing a series of nodular lesions. Trematodes live in the host up through much of its adult life, producing more severe symptoms with increasing age. Symptoms generally include diarrhea, convulsions, and loss of appetite. The trematodes eventually kill the host. Other trematodes (Echinoparyphium oscitansi) have also been found in the intestinal nodules of Asian openbills. Their pathology in Asian openbills has not been well-studied. (Acharjyo, 1970; Chatikavanij, 1989)

Commensal/Parasitic Species
  • trematode (Chaunocephalus ferox)
  • trematode (Echinoparyphium oscitansi)

Economic Importance for Humans: Positive

Asian openbills produce feces that serve as fertilizer for wetland plants, leading to increases in both the plants and fish and crab populations which feed on them. Fishermen benefit from the abundance of such organisms.The meat and eggs of Asian openbills are regarded as delicacies, and are sold at high prices in the market, enabling poachers to earn substantial profit. Asian openbills also feed on golden apple snails, which are major rice pests in Asia, effectively reducing their populations. (Sin, 2003; Nayak, 2012; Bock, 2002)

  • Positive Impacts
  • food
  • produces fertilizer
  • controls pest population

Economic Importance for Humans: Negative

Asian openbills carry and transmit H5N1, a form of avian influenza. It is questionable whether Asian openbills have the ability to transmit H5N1 directly to humans. Researchers have hypothesized that this is quite unlikely, as Asian openbills tend to keep a great distance from human civilization. They can contract the virus by coming in contact with fecal matter of infected birds. Once infected, Asian openbills house the virus for long periods of time and transmit it to smaller terrestrial birds which come in contact with poultry frequently. In turn, humans who come in contact with infected poultry are highly susceptible to the virus. However, again researchers that they are an unlikely vector for transmission. (Hayashi, 2008; Hubalek, 2006)

Conservation Status

Asian openbills are largely unthreatened, however several threats exist which can potentially cause their populations to decline. Invasive weed species can choke wetlands, resulting in reduced water flow and drying of wetland habitat. Large animals such as buffaloes tend to destroy wetland habitats and consume many of their resources. Fishing further reduces food sources for Asian openbills. Pesticides used by farmers in agricultural wetlands can increase mortality among members of this species. In addition, farmers use rockets, polythene bags and other harmful devices to scare away storks. Asian openbills are often victims of poaching, which can have detrimental impacts on population size. Reclamation of wetlands by the government for developmental purposes also poses a major threat. (Jayson, 2002; Nayak, 2012)

In recent years, several measures have been taken in an effort to conserve Asian openbills. Strict laws have been implemented which prohibit poaching and fishing in wetland areas. The government has also strived to increase public awareness by offering educational classes and transforming wetland reserves into eco-tourism sites. Various conservation committees have been established by former poachers, who have been successful in recruiting other poachers by promising them an alternate source of income. More stringent regulation of developmental undertakings has also been implemented to increase the effectiveness of conservation. (Jayson, 2002; Nayak, 2012)


Sanna Quasmieh (author), The College of New Jersey, Matthew Wund (editor), The College of New Jersey, Laura Podzikowski (editor), Special Projects.



uses sound to communicate


young are born in a relatively underdeveloped state; they are unable to feed or care for themselves or locomote independently for a period of time after birth/hatching. In birds, naked and helpless after hatching.

bilateral symmetry

having body symmetry such that the animal can be divided in one plane into two mirror-image halves. Animals with bilateral symmetry have dorsal and ventral sides, as well as anterior and posterior ends. Synapomorphy of the Bilateria.

brackish water

areas with salty water, usually in coastal marshes and estuaries.


an animal that mainly eats meat

causes or carries domestic animal disease

either directly causes, or indirectly transmits, a disease to a domestic animal


uses smells or other chemicals to communicate


used loosely to describe any group of organisms living together or in close proximity to each other - for example nesting shorebirds that live in large colonies. More specifically refers to a group of organisms in which members act as specialized subunits (a continuous, modular society) - as in clonal organisms.

  1. active during the day, 2. lasting for one day.

animals that use metabolically generated heat to regulate body temperature independently of ambient temperature. Endothermy is a synapomorphy of the Mammalia, although it may have arisen in a (now extinct) synapsid ancestor; the fossil record does not distinguish these possibilities. Convergent in birds.


an area where a freshwater river meets the ocean and tidal influences result in fluctuations in salinity.

female parental care

parental care is carried out by females


A substance that provides both nutrients and energy to a living thing.


mainly lives in water that is not salty.


offspring are produced in more than one group (litters, clutches, etc.) and across multiple seasons (or other periods hospitable to reproduction). Iteroparous animals must, by definition, survive over multiple seasons (or periodic condition changes).

male parental care

parental care is carried out by males


marshes are wetland areas often dominated by grasses and reeds.


makes seasonal movements between breeding and wintering grounds


eats mollusks, members of Phylum Mollusca


Having one mate at a time.


having the capacity to move from one place to another.

native range

the area in which the animal is naturally found, the region in which it is endemic.


found in the oriental region of the world. In other words, India and southeast Asia.

World Map


reproduction in which eggs are released by the female; development of offspring occurs outside the mother's body.


having more than one female as a mate at one time


reproduction that includes combining the genetic contribution of two individuals, a male and a female


associates with others of its species; forms social groups.


uses touch to communicate


defends an area within the home range, occupied by a single animals or group of animals of the same species and held through overt defense, display, or advertisement


the region of the earth that surrounds the equator, from 23.5 degrees north to 23.5 degrees south.


uses sight to communicate


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