Identify the fauna from the options based on the following information thereof.
It is one of the heaviest flying birds in the world. It is a tall bird with long legs and a long neck. Feathers on the top of the head are black in males whereas females possess a smaller black crown on the top of the head. It inhabits dry grasslands and scrublands on the Indian subcontinent. Its largest populations are found in the Indian state of Rajasthan. By 2011, its population decline was so severe that the IUCN classified the species as critically endangered.