River tern

It is a medium-large tern, 38–46 cm long with an 80–85 cm wingspan, distinctly larger than any other species in Sterna and more comparable in size to a Sandwich tern (Thalasseus sandvicensis) or a lesser crested tern (T. bengalensis). It 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. The bill is stout for a tern, bright yellow in summer and duller yellow with a dusky tip in winter, and the legs bright red. It has a black cap in breeding plumage, which is more extensive than in other Sterna terns, extending well below the eye instead of barely or not below the eye. In winter the cap is greyish white, flecked and streaked with black, with a dark mask through the eye; the two longest outer tail feathers are also lost in winter, making it conspicuously shorter-tailed then. The sexes are similar but juveniles have a brown head, brown-marked grey upperparts, grey breast sides and white underparts, and the bill yellowish with a dark tip. ==Taxonomy==

Its placement in Sterna is uncertain, based only on its appearance and behaviour; it has yet to be tested genetically to determine its closest relatives. In the IOC World Bird List, it is placed as basal in the genus Sterna. It is monotypic, with no geographical variation. ==Ecology==

The river tern is typically a lowland riverine freshwater species, mainly occurring from 0–600 m altitude, It also uses freshwater reservoirs extensively, which has allowed for local increases in population in some areas against the general trend of declines. It has been recorded as a vagrant in Afghanistan and Iran. ==References==