The Greater Crested Tern (Thalasseus bergii) is a large tern with a long (5.4–6.5 cm) yellow bill, black legs, and a glossy black crest that is noticeably shaggy at its rear.
The Greater Crested Tern often associates with the Lesser Crested Tern (Thalasseus bengalensis), but is 25% larger than the latter, with a proportionately longer bill which is yellow and not orange-tinted like in Lesser Crested Tern.
The Royal Tern (Thalasseus maximus) is similar in size to the Greater Crested Tern, but has a paler back and a blunter, more orange bill.
The Greater Crested Tern is a widespread tern found along the coasts of Africa, the Indian ocean, south eastern Asia and Australia.

1st, 2nd & 3rd photo: In the company of Heuglin's Gulls, a Lesser Crested Tern (orange bill) and a Saunders' Tern (black bill). Mughsayl Beach area, Dhofar Region, Oman, 3 February 2017
4th photo: Salalah Beach, Dhofar Region, Oman, 2 February 2017