Asian Openbill

The Asian openbill (Anastomus oscitans) is a medium-sized stork of South and Southeast Asian wetlands. Adults are roughly 76–81 cm long, have a wingspan of about 147–149 cm and weigh around 1.35 kg. The plumage is pale grey-white to white with contrasting black flight feathers and tail, while young birds are browner. The long legs and broad wings are typical of a stork adapted to wading in shallow water and travelling efficiently between wetlands.

The only other species in the genus is the African openbill (Anastomus lamelligerus), which is predominantly dark and occurs in sub-Saharan Africa. The two share the characteristic open-billed form but are geographically separate.

Asian openbills are wetland specialists. They use shallow marshes, floodplains, ponds, lakeshores, flooded grassland and slow-moving freshwater, as well as rice fields and other seasonally inundated agricultural areas. Productive shallow water is especially valuable because it supports the large freshwater snails on which the species depends.

The birds also need suitable trees for roosting and breeding. Colonies may occur in woodland managed by local communities, farmland or even within settlements when large mature trees are retained close enough to feeding wetlands. This combination of productive wetlands and large nesting trees is therefore more important than whether the surrounding landscape is entirely natural.

Large freshwater snails form the core of the diet, including native apple snails and, in parts of Southeast Asia, the invasive golden apple snail. Other aquatic invertebrates, large insects, frogs and occasional fish are also taken.

Foraging usually takes place in shallow water, where birds walk slowly while searching visually and by probing. Dense concentrations of snails can attract sizeable feeding groups, linking the species' specialised diet with its strongly social use of productive wetlands.

The conspicuous gap between the upper and lower mandibles develops as the bird matures; juveniles have a much straighter bill without the pronounced opening. The unusual shape is closely associated with the specialisation on freshwater snails, but it is not simply a built-in shell opener.

A study published in 2021 from rice fields in Thailand documented openbills puncturing the shells of invasive golden apple snails before extracting the soft body. The birds sometimes swallowed whole snails including shell and operculum. These observations show that snail handling is more varied than a single fixed technique and that both the bill gap and the narrow bill tip form part of a highly specialised feeding apparatus.

Asian openbills regularly use rising warm air to soar between feeding and resting areas with little energetic cost. They can also tolerate considerable human presence and may continue moving and feeding close to settlements where suitable wetlands and safe trees remain available.

Asian openbills are strongly social. They frequently forage in loose to dense flocks where snail concentrations are high and gather at communal roosts. Outside breeding colonies, flock size changes with local food conditions and the availability of suitable resting sites.

Breeding is closely linked to seasonal rainfall and wetland productivity and often peaks during the monsoon period. Large stick nests are built high in trees, usually as part of conspicuous colonies that may contain hundreds or even thousands of nests. Clutches usually contain two to five eggs. Both parents incubate for about 27–30 days and later care for the chicks, which normally leave the nest after roughly 35–36 days.

A three-year study published in 2024 monitored 67 active colonies containing 4,020 active nests in eastern Nepal from 2020 to 2022. Forty per cent of the colonies were inside settlements, 33% in woodland managed by local communities and 27% in agricultural land. Silk-cotton trees (Bombax ceiba) accounted for 72% of nesting trees. Larger trees with broader canopies supported larger colonies, and larger colonies were associated with greater fledging success; in total, 7,566 young fledged during the study. The results show how strongly large, mature trees can support successful breeding even in heavily used wetland landscapes.

The Asian openbill is classified as least concern on the IUCN Red List and remains widespread across South and Southeast Asia. State of India’s Birds 2025 estimates a long-term national increase of about 90.33%, with an uncertainty interval of 65.71–120.84%. The recent annual estimate is about −0.58%, but its interval of −1.44 to +0.28% crosses zero, so there is no clear recent national change.

Local colonies can nevertheless disappear quickly when nesting trees or nearby wetlands are lost. A 2024 study along the Keleghai River in India documented contraction of breeding colonies alongside major land-use change and loss of large trees around colony sites. The species can therefore remain common at a broad scale while depending strongly on very specific local habitat features.