Which of the following is not a bird ?

The correct option is Golden Mahseer.

To understand why this is the correct answer, let's look at each of the choices provided:
1. Golden Mahseer (Tor putitora): This is actually an endangered species of cyprinid fish found in rapid-flowing rivers of the Himalayan region, often referred to as the "tiger of the water". Since it is a fish, it is not a bird.
2. Indian Nightjar (Caprimulgus asiaticus): This is a small nocturnal bird species native to southern Asia.
3. Spoonbill: This is a large, long-legged wading bird characterized by its distinctive flat, spatula-like bill.
4. White Ibis: This is a wading bird belonging to the ibis family, easily recognizable by its long, curved bill.

Therefore, among the given options, the Golden Mahseer is the only one that is not a bird.