Created in 1973 and constituted as a Reserve Forest in 1978, the Sunderbans was established as a National Park in 1984, listed as a UNESCO Natural World Heritage Site in 1985 and declared a Biosphere Reserve four years later. The Sunderbans (sometimes spelled Sundarbans) is part of a vast estuary in the Bay of Bengal. Its mangrove forests are the only tiger habitat of its kind, with an estimated 270 of the big cats found there – these animals might represent a distinct subspecies of tiger. The trees and other wildlife have adapted to the estuarine conditions of high salinity, lack of soil, erosion and daily inundation by high tides. Fishing cat, spotted deer, wild boar, water monitor, estuarine crocodile, river terrapin, olive Ridley turtle, ground turtle, king crab, rhesus monkey and mud skipper can be seen there as well as 230 or so bird species. These include red jungle fowl, lesser whistling-duck, cotton pygmy-goose, streak-throated woodpecker, coppersmith barbet, brown-winged, stork-billed, black-capped, pied and collared kingfishers, Brahminy kite and white-bellied fish eagle.