Kaziranga National Park
Kaziranga's wetlands and grasslands sit along the Brahmaputra river floodplain and host over 478 species of birds — including the rare Pallas's fish eagle, bar-headed geese that migrate over the Himalayas, and the endangered greater adjutant stork. For serious birders, this is one of the finest wetland birding destinations in the entire Indian subcontinent, rivalling even Bharatpur at its peak.
The Diphlu River area and the Mihimukh area in the western range are the most productive birding spots. A good field guide to Northeast Indian birds is worth bringing.