Black-bellied Tern

Overview

The black-bellied tern (Sterna acuticauda) is a tern found near large rivers in the Indian subcontinent, its range extending from Pakistan, Nepal and India to Myanmar. It has become very scarce in the eastern part of its range and the International Union for Conservation of Nature has assessed its conservation status as being endangered.
They have a black belly in the summer and a deep forked tail. They can sometimes resemble whiskered terns (Chlidonias hybrida), but the deeper fork of the tail and the black on the lower belly distinguish them from the shallow fork and black closer to the breast on the whiskered tern.

Summary from Wikipedia, available under CC BY-SA 4.0.