African Openbill

The African openbill (Anastomus lamelligerus) is a dark, medium-sized stork of wetlands across sub-Saharan Africa and Madagascar. Adults are roughly 80–94 cm long and weigh about 1.0–1.2 kg, with males averaging heavier than females. The plumage appears black at a distance but can show greenish or purplish gloss in good light. The long legs are dark and the large grey-brown bill is unmistakable.

In adult birds the mandibles do not meet along much of their length, creating the characteristic gap that gives openbills their name. Juveniles begin life with a much straighter and more conventional bill, and the specialised adult shape develops as they grow and begin handling molluscs.

The genus Anastomus contains only two living species. The other is the Asian openbill (Anastomus oscitans), which has much paler plumage and occurs in Asia.

African openbills are strongly associated with shallow freshwater habitats such as marshes, floodplains, lake margins, swamps, rice fields and seasonally inundated grassland. Productive wetlands with fluctuating water levels are especially important because flooding can create temporary concentrations of aquatic snails and other molluscs.

Numbers at a site can therefore change dramatically between seasons and years. Birds readily move between feeding areas as water levels and prey abundance shift, and large aggregations can form wherever conditions temporarily become favourable.

The diet is highly specialised and dominated by aquatic snails, supplemented by freshwater mussels, terrestrial snails and occasional frogs or other invertebrates.

African openbills usually walk slowly through shallow water and mud while locating prey by sight and touch. After flooding, snail populations can increase sharply, and the birds may then gather in large numbers to exploit the temporary abundance.

The unusual bill is specialised for handling slippery, hard-shelled prey, but it does not function as a shell-crusher. Classic field studies of both openbill species showed that the converging tips of the mandibles help grip and orient molluscs. The bird positions a snail carefully, inserts the narrow tip of the lower mandible through the shell opening and severs or loosens the muscle anchoring the animal inside before extracting the flesh.

The gap itself may help the bill grip and handle snails, although its exact function has long been debated. What is clear is that the adult bill allows rapid extraction of soft tissue without first breaking the shell, making openbills exceptionally efficient snail specialists.

African openbills are monogamous colonial nesters. Breeding is usually linked to rainfall, flooding and abundant snail prey. Both sexes build a stick platform in trees, bushes or dense wetland vegetation and share incubation and chick care. Clutches commonly contain three or four eggs, incubation lasts roughly 25–30 days and the young leave the nest after about 50–55 days.

Some colonies are extraordinarily large. A 2023 survey of the Barotse Floodplain in western Zambia estimated about 70,000 African openbill nests in three reedbed colonies, with roughly 57,900 nests in the largest colony alone. In the same study, a male fitted with the first telemetry transmitter deployed on the species made 73 recorded feeding trips averaging 16.4 km round-trip from the colony, showing how widely breeding birds may range between nesting and feeding sites.

The African openbill is classified as least concern on the IUCN Red List. It remains widespread and can be locally extremely numerous when wetland conditions produce abundant mollusc prey.

Wetland drainage, altered flooding regimes, pollution and disturbance can reduce local feeding habitat or disrupt breeding colonies. The enormous colonies documented in Zambia also show how a relatively small number of productive floodplain sites can become exceptionally important to the species at particular times.