River Tern

The river tern (Sterna aurantia) is a medium-sized freshwater tern of South and Southeast Asia, measuring about 38–46 cm with a wingspan of roughly 80–85 cm. Breeding adults have a black cap, pale grey upperparts, white underparts, a long deeply forked tail and a bright yellow to orange-yellow bill. Outside breeding the forehead becomes paler and the cap less complete.

The endangered black-bellied tern (Sterna acuticauda) shares some of the same river systems but is smaller and slimmer, with a shorter darker bill and a black belly in breeding plumage. River tern is the larger, paler bird with the conspicuous yellow bill.

River terns are strongly associated with large inland rivers, lakes and reservoirs. They feed over open freshwater and depend on exposed sandbanks, gravel bars, rocky islands and low river islands for breeding and resting. Estuaries and coastal lagoons are used locally, but the species is much more closely tied to inland waters than most terns.

Suitable breeding habitat is naturally temporary. Falling dry-season water levels expose islands and bars, while later flooding or reservoir releases can submerge them. Breeding success therefore depends strongly on suitable water levels through the nesting period and access to fish nearby.

Small fish form most of the diet, supplemented by crustaceans, aquatic insects, tadpoles and other small aquatic prey. During breeding, adults repeatedly carry prey back to the colony to feed their chicks.

River terns patrol above open freshwater while scanning the surface for prey. They often pause in a brief hover before plunge-diving, sometimes submerging the whole body for an instant before reappearing with a fish. Shallower dips and surface captures are also used when prey is already close to the top of the water.

A field note published in 2019 from Gujarat documented a river tern repeatedly following little cormorants and diving beside them as the cormorants fed, apparently taking fish driven toward the surface.

River terns occur alone, in pairs or in small flocks and often rest together on exposed sandbars, rocks and islands. They are mainly active by day and may travel considerable distances between feeding waters and resting or breeding sites.

Most populations are resident or make local seasonal movements as water levels and fish availability change. Birds also associate with other terns and waterbirds on productive rivers and reservoirs.

Breeding is concentrated in the dry season and early summer, before monsoon rains raise water levels. Colonies are established on exposed sand, gravel or rocky islands and riverbanks, and the nest is a shallow scrape with little or no lining. Clutches usually contain two or three camouflaged eggs.

A three-year study along the Mahanadi River in Odisha, published in 2022, found a mean incubation period of 22.1 days. Both parents incubate and defend the immediate nest area with sharp calls and aerial attacks. The chicks hatch mobile and soon leave the nest scrape, but remain dependent on their parents for food and protection.

The same study found large year-to-year variation in nesting success, from 2.6% to 62.3%. Predation, flooding and a cyclone caused most known nest failures. Overall, 124 chicks hatched and 58 were estimated to fledge; a cyclone in May 2019 destroyed all active eggs and chicks at the study sites.

The river tern is classified as vulnerable on the IUCN Red List, with a decreasing global population trend. The species remains widespread, but many river and reservoir breeding populations are small, fragmented or poorly monitored.

State of India's Birds 2025 places the river tern in the Moderate Priority category. Its updated national estimate indicates a long-term decline of about 15.6%, while the current annual estimate is a small increase of about 0.7%. Uncertainty around both estimates is substantial, particularly for the recent trend.

Dams, river regulation and reservoir management can remove exposed nesting islands or flood colonies through sudden water releases. Sand and gravel extraction, conversion of riverbanks, pollution and declining fish stocks further alter feeding and breeding habitat.

Ground nests are also vulnerable to repeated disturbance, trampling, fishing activity, dogs and natural nest loss. Water-level changes, disturbance and predation can all reduce breeding success.

A five-year study on the Mekong River in Cambodia showed how strongly nest protection can affect breeding success. River tern nests protected by both local guards and simple predator-exclusion fencing had about 60% nest success, compared with 29% for guarded nests without fencing and 5% for unprotected nests. Fledging success showed the same pattern: about 82%, 40% and 2%, respectively.

In India, river tern is included in the National Mission for Clean Ganga programme for sandbar-nesting birds. The project combines breeding surveys, local nest guardians and coordination with water-resource authorities around disturbance, livestock impacts, sand mining and harmful water-level changes along the Ganga and its tributaries.