The open-billed stork is one of Africa’s most distinctive and specialised waterbirds. With their unique bill that remains open even when closed — an adaptation for cracking mollusc shells — these grey waders are a fascinating sight in East Africa’s wetlands. Their specialised diet and tool-using behaviour make them particularly interesting for birding enthusiasts. Whether you are planning a Tanzania safari or Kenya safari, this guide covers everything you need to know about these remarkable mollusc specialists.
Key Highlights
- Unique bill: gap remains open even when bill is closed
- Specialised mollusc hunter: uses bill as a tool to crack shells
- Size: 80 to 100 cm, wingspan 1.3 to 1.5 metres
- Grey plumage with glossy green-purple sheen on wings
- Best locations: Serengeti wetlands, Lake Victoria, Tana River
- Best viewing: year-round at wetlands and river systems
Why Open-Billed Storks Are Special
The open-billed stork (Anastomus lamelligerus) is one of Africa’s most distinctive waterbirds. Their most remarkable feature is their bill — the upper and lower mandibles curve towards each other but do not meet at the tip, leaving a characteristic gap even when the bill is closed. This gap is perfectly adapted for their specialised diet of aquatic snails and mussels. The stork inserts its bill into the shell, uses the gap to grip the mollusc’s body, and then snaps the bill shut with enough force to extract the prey. They have been observed using stones and sticks as tools to break open particularly tough shells — a rare example of tool use among birds. Open-billed storks are gregarious, often feeding in flocks and breeding in colonies alongside other waterbirds.
Where to See Open-Billed Storks
Tanzania: The Serengeti’s wetlands and river systems support open-billed storks, particularly during the wet season when mollusc populations expand. Lake Victoria’s shores have good numbers.
Kenya: The Tana River Delta is one of the best places to see open-billed storks in Kenya. Lake Naivasha and Lake Nakuru have populations. The Masai Mara’s river systems attract storks during flooding.
Behaviour and Ecology
Open-billed storks feed primarily on aquatic snails and freshwater mussels, using their specialised bill to extract prey from shells. They walk slowly through shallow water or wet grassland, probing for molluscs with their bill. When a mollusc is located, they grip it with the bill gap and use a rapid shaking motion to extract the body. For particularly tough shells, they have been observed using stones and sticks as hammers — a remarkable example of tool use. They are highly social, feeding in flocks that can number hundreds of birds during peak food abundance. Breeding colonies are established in trees near water, with nests built from sticks. Both parents incubate and feed the chicks.
Frequently Asked Questions
Where is the best place to see open-billed storks?
Open-billed storks are found throughout East Africa’s wetland systems. The Tana River Delta in Kenya is one of the best locations, with large populations feeding on the river’s abundant molluscs. In Tanzania, the Serengeti’s wetlands during the wet season (November to May) provide excellent viewing as storks follow expanding water and mollusc populations. Lake Victoria’s shores support year-round populations. Lake Naivasha and Lake Nakuru in Kenya are reliable sites. Because they are tied to wetland habitats, any safari that includes lake or river environments is likely to produce sightings, particularly during or after the rainy season when mollusc populations are highest.
Why do open-billed storks have a gap in their bill?
The gap in an open-billed stork’s bill is a remarkable evolutionary adaptation for their specialised diet of aquatic snails and mussels. The upper and lower mandibles curve towards each other but do not meet at the tip, creating a gap that is perfectly sized to grip the body of a mollusc inside its shell. When feeding, the stork inserts its bill into the shell opening, grips the mollusc’s body with the gap, and then snaps the bill shut while shaking the head rapidly. This motion extracts the prey from the shell. The gap also allows them to carry large molluscs without the shell edges cutting into the bill. This specialised bill makes them one of the most perfectly adapted mollusc hunters in the bird world.
Do open-billed storks use tools?
Yes, open-billed storks have been observed using tools — a rare behaviour among birds. When they encounter molluscs with particularly tough shells that cannot be opened with their bill alone, they use stones and sticks as hammers to break the shells. The stork picks up a stone in its bill and strikes the shell repeatedly until it cracks. Some individuals have been observed using the same stone repeatedly, suggesting learned behaviour. Tool use in birds is extremely rare and is considered evidence of advanced cognitive ability. Open-billed storks are one of only a handful of bird species worldwide known to use tools, alongside crows, gulls, and a few parrot species.
What do open-billed storks eat besides molluscs?
While aquatic snails and freshwater mussels make up the majority of their diet, open-billed storks are opportunistic and will take other prey. They eat frogs, small fish, aquatic insects, and crustaceans. They have been recorded catching land snails after rains and occasionally take small reptiles and amphibians. When mollusc populations are low, they switch to alternative prey. However, their bill is optimised for mollusc extraction, so they are less efficient at catching other prey types. During the dry season, when wetlands shrink and molluscs become concentrated, they can feed almost exclusively on their preferred prey, achieving remarkable feeding success rates.
When is the best time to see open-billed storks?
Open-billed storks are present year-round in East Africa but are most visible during and after the wet season (November to May). During this period, expanding wetlands create ideal mollusc habitat, and storks gather in large feeding flocks. The dry season (June to October) sees them more concentrated around permanent water sources. Early morning and late afternoon are the best times for viewing, as storks are most actively feeding during cooler hours. Breeding season varies by location but often coincides with peak water levels. Because they are gregarious and feed in open wetlands, they are relatively easy to spot during game drives in appropriate habitat.
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