Sterna aurantia J.E.Gray, 1831

Common Name and Taxonomy

Sterna aurantia, commonly known as the river tern, is a medium-large tern species.

Size Measurements

It measures 38–46 cm in length with an 80–85 cm wingspan, and is distinctly larger than all other species in the Sterna genus, with a size more comparable to the Sandwich tern (Thalasseus sandvicensis) or the lesser crested tern (T. bengalensis).

General Plumage and Body Features

This tern has dark grey upperparts, white or very pale grey underparts, a forked tail with long flexible streamers, and long pointed wings with pale grey primaries.

Bill Characteristics

Compared to other terns, its bill is stout; it is bright yellow in summer, and duller yellow with a dusky tip in winter.

Leg Coloration

Its legs are bright red.

Breeding Plumage Cap

In breeding plumage, it has a black cap that is more extensive than the cap of other Sterna terns, extending well below the eye rather than barely or not reaching below the eye.

Non-breeding Plumage Head Features

In winter, the cap becomes greyish white, flecked and streaked with black, and it develops a dark mask through the eye.

Winter Tail Morphology

The two longest outer tail feathers are also shed in winter, resulting in a conspicuously shorter tail during this season.

Juvenile Plumage

Males and females have similar plumage, but juveniles differ: they have a brown head, grey upperparts marked with brown, grey breast sides, white underparts, and a yellowish bill with a dark tip.

Typical Habitat and Altitude Range

The river tern is typically a freshwater species found in lowland river environments, mainly occurring at altitudes between 0 and 600 m.

Exceptional Altitude Record

Exceptionally, it has been recorded at 2,706 m altitude in the Upper Indus valley in the western Himalaya.

Reservoir Habitat Use

It also makes extensive use of freshwater reservoirs; this has allowed local population increases in some areas, against the overall general trend of population decline for the species.

Marine Avoidance

The river tern is reluctant to use or cross the sea, which is demonstrated by the complete lack of records of the species from Sri Lanka, even though it is widespread across Kerala and Tamil Nadu in southernmost India.

Breeding Period and Colony Location

This species breeds from March to May in colonies located in less accessible areas such as river sandbanks.

Nesting and Egg Characteristics

It nests in a scrape dug into the ground, often on bare rock or sand, and lays three greenish-grey to buff eggs marked with brown blotches and streaks.

Diet and Feeding Behavior

Like other Sterna terns, the river tern feeds by plunge-diving for fish, crustaceans, tadpoles, and aquatic insects in rivers, lakes, and tanks.

Population Trend and Threats

Its overall population is decreasing, driven by commercial river development, human disturbance, and pollution of its habitat.

Regional Conservation Status

It is particularly endangered in the eastern portion of its range: in far southern China, only one breeding site remains, and the species is also endangered in Cambodia, Laos, and Thailand.

Vagrant Records

It has been recorded as a vagrant in Afghanistan and Iran.