Asian Openbill

BirdAsia

Nov 22

Anastomus oscitans

The Asian openbill uses its "open bill" to locate and grab its favourite prey — freshwater snails — using the sharp tip of its curved, lower mandible to extract them from their shells. It is a common species of stork throughout South and Southeast Asia.

A large bird glides into view over flooded rice fields. Its wings — some metre and a half (4.9 ft) from tip to tip — ride the warm airstream. Its gangly, pink legs trail behind it and its long neck is held straight ahead. Its feathers are ashen grey, but its wings are black as soot. This great roc descends to land, its legs swing forward and dangle, as its wings ferry it down like a parachute. Grounded now amidst green stalks of rice, the bird cries out (“hoo-hoo”) and clacks its bill in exaltation; a hollow sound, for its mandibles split not far from their base, as the lower one dips into a curve, and only reunite once more at the very tip of its bill. This "open bill" is the distinguishing feature and namesake of Anastomus oscitans, more commonly known as the Asian openbill.

Gastropod Gourmand

From South Asia (India and Sri Lanka) through to the Southeast (to the tip of the Malay Peninsula), this great grey bird wades through wetlands and lakes, flooded fields and park ponds. It stays dry on its stilt legs, strolling languidly through waters anywhere from 10 to 50 centimetres (∼4 - 20 in) deep and it relishes in rainfalls — more rain means more wetlands and more wetlands means more food.

The openbill is a carnivore with a gastronomic predilection for gastropods. Its favourite prey is freshwater snails, more specifically apple snails (in the family Ampullariidae) — the snails that stick clusters of bright pink eggs onto vegetation. With its bill slightly ajar, this bird probes beneath the water's surface with rapid jabs, using touch to find its quarry within the muck — successfully doing so even when blindfolded. Having found a snail, it grasps the slippery shell between the rough-textured edges of its bill, with the odd opening perhaps providing additional grip on the globular mollusc. The openbill's lower mandible, sharp-tipped like a bent spear, is stabbed into the shell, extracting the tasty snail meat with a twist — if the snail is small enough, however, it is simply crushed, and the entire thing is eaten. If the openbill can't dine on escargot, it'll also take a dish of crab, or perhaps a feast of frog or fish.

Arboreal Apartments

The Asian openbill is rarely seen dining alone; a result of its decidedly social disposition. These birds travel and nest together, forming large colonies comprised of up to 150 nests spread out between several trees. They nest alongside other species, such as herons, egrets, and cormorants, and each tree is like a green condominium, stratified based on each resident’s needs. The openbills, for example — with their tall nests, each about a metre in length (3.3 ft) — occupy the uppermost canopy layer of a tree, while birds with smaller nests live in the boughs below. Any predators — spotted eagles, crows, or monitor lizards — hoping to dine on eggs or chomp some chicks, are met with the fierce resistance of an entire tree community; warded off by the jabbing bills of heron and openbill parents, making for a fairly safe environment in which to raise a family.

Not-So-Nuclear Families

Asian openbills are mostly monogamous. Arriving before the singles, devoted couples have the advantage of first pick on nesting sites. Males show off potential sites to their partners and flourish their building skills by picking up and manipulating different construction materials — branches, grasses, and leaves. Having been satisfactorily impressed, females help their partners with the actual construction of the nest.

But not every openbill is willing to settle for one partner and often, it's not predators, but other openbills that spoil the domestic bliss of a nest. Single male openbills arrive at the nesting trees after the established pairs have already claimed their nest sites. Seeing no vacancies, the lone males don't accept their bad luck and wait til next year, instead, they attempt to force their way into existing relationships. Unsurprisingly, fights break out. Established males fight invading males, attracting other single males — no doubt rowdy with hormones — to join in the brawl. These altercations can end in one of several ways. Sometimes, the original male will fend off the homewrecker. Sometimes the new male will claim the nest and the female for himself. But, on rare occasions, both the original and arriving males will settle at the same nest site, equally sharing nest-building, incubation, and child-rearing responsibilities.

Clattering Bills & Cooperative Parenting

Before a couple can mate, there is a lot of flying about together, but the whole affair is mostly mute. There is no honking as in swans, nor a back-and-forth duet as in owls — at most, an openbill pair (or trio) will utter a mournful sounding "hoo-hoo", for openbills lack the syrinx muscles necessary for vocal control. Instead, they use their boomerang-bills as instruments, sounding like drummers with no drums, which is to say, like wooden sticks being clacked together.

A young Asian openbill, its lower mandible yet to start curving.

Openbill breeding season arrives sometime between June and December, after the rains have swept through and left the lands lush — a drought might mean no breeding that year. The female lays 2 to 5 white eggs in a treetop nest and both sexes incubate the clutch for up to a month. As the helpless hatchlings break free from their eggs, the parents fastidiously remove eggshell pieces from the nest. During this vulnerable period, both parents remain at the nest to watch over their young, safeguarding them from both predators and the weather, but after another month or so, once the chicks have fledged, the parents begin to take shifts — one foraging and one babysitting. At only two months old, the young are considered sexually mature and strike out on their own; already, they stand tall on spindly legs, with heavyset bodies and grey feathers not yet fully darkened along their wings. Their bills are like straight spears of dark ivory, formed from two unbent mandibles which will gradually "open" as the openbills grow older.

An Odd Order of Birds

The Asian openbill, seen from a distance amidst a colony of other lanky birds, is often mistaken for a heron — it has the proportions and the colours. But the openbill is actually a member of the order Ciconiiformes; some twenty species of birds that are commonly known as storks.

The white stork is perhaps the most famous, because of its vast range (covering most of Europe, Africa, and some of Asia) but more so for its storied job as a baby deliverer. The Americas are home to the wood stork; a slightly less comely bird with a naked vulturine head, that ranges from the southeastern United States down to Argentina. While Australia receives migrating populations of stunning black-necked storks.

There are beautiful storks, like the black stork with its dark feathers that shimmer like an oil slick and the endangered oriental stork with its monochrome plumage and sky-blue eyes streaked with red. There are also (to put it gently) unsightly storks, like the milky stork, whose face looks both like a balding man's and somewhat like a scrotum, or the marabou stork with its naked, sunburnt-looking head and dangling bulge of loose skin at its throat. And then there are the storks that are just plain weird, including the saddle-billed stork, which competes for the most striking bill of any bird, and the jabiru, which might pass for a normal wading bird until it inflates its black and red throat sac like a balloon.

Compared to some of its relatives, the Asian openbill's gap-billed appearance seems much less peculiar. It shares the trait with its close cousin from the wetlands of Sub-Saharan Africa. The African openbill opts for an all-black appearance, but otherwise, the two openbill species have much in common — including an unmatched love for snails.

Clockwise from top; white stork (*Ciconia ciconia*), wood stork (*Mycteria americana*), and black-necked stork (*Ephippiorhynchus asiaticus*).

Clockwise from top left; black stork (*Ciconia nigra*), oriental stork (*Ciconia boyciana*), milky stork (*Mycteria cinerea*), and marabou stork (*Leptoptilos crumenifer*).

A saddle-billed stork (*Ephippiorhynchus senegalensis*) on the left and a jabiru (*Jabiru mycteria*) on the right.

An African openbill (*Anastomus lamelligerus*) — the only other openbill species.

**Where Does It Live?**

⛰️ Wetlands, rivers, fields, and urban parks.
📍 South and Southeast Asia.

‘Least Concern’ as of 01 October, 2016.

IUCN

Size // Medium
Wingspan // 147 - 149 cm (58 in)
Length // 76 - 81 cm (30 - 32 in)
Weight // 1.3 to 8.9 kg (2.9 - 19.6 lbs)
Activity: Diurnal ☀️
Lifestyle: Social 👥
Lifespan: Up to 18 years in captivity
Diet: Carnivore
Favorite Food: Freshwater snails 🐌
Class: Aves
Order: Ciconiiformes
Family: Ciconiidae
Genus: Anastomus
Species: A. oscitans

The Asian openbill is a social bird, forming large colonies of up to 150 nests spread out between several trees.

A large nesting tree can be like a green condominium comprised of different birds. Open bill nests, each about a metre in length (3.3 ft), are the penthouse suites — built in the upper canopy layer. In the branches below nest birds like herons, egrets, and cormorants. Any predator that tries to invade the tree is met with an onslaught of pecking and jabbing from different species.

Asian openbills are mostly monogamous — mostly. Young single males will sometimes try to butt in on established couples. If a young male is successful, he either drives the old male away, claiming the nest and female, or, on some occasions, three openbills share the responsibility of raising a clutch.

The Asian openbill isn't a very vocal bird — its call is a mournful sounding "hoo-hoo". But it does clatter its bill to communicate.

The Asian openbill's vast range extends from India and Sri Lanka, down to mainland Southeast Asia and the tip of the Malay Peninsula.

It wades through wetlands, lakes, flooded fields, and park ponds. Its stilt-like legs carry it through water anywhere from 10 to 50 centimetres (∼4 - 20 in) deep.

Like other storks, it flies with its neck held straight forward and its gangly pink legs trailing behind it — soaring on thermals with slow flaps of its great wings that measure a total span of 1.5 metres (4.9 ft). As it lands, it swings its legs forward and descends steadily.

The Asian openbill is somewhat of a gastropod gormand — with a particular liking for apple snails (in the family Ampullariidae). It uses the tip of its curved lower mandible to stab into a snail shell, gives it a twist, and extracts the slimy morsel.

It finds snails by probing with its bill beneath the water — succeeding even when blindfolded. The edges of its bill are rough for gripping slippery shells while its bill gap may help it to hold onto a snail, or serve some other unknown purpose.

Juvenile openbills lack an open bill. As they age, their bills grow longer, and the lower mandible becomes more and more curved.

The only other openbill species is the African openbill; an all-black stork with a similarly gaped bill.