Caspian Terns look similar to Royal Terns and they occur sometimes in the same place.
With these two species, the most important characteristics are the bill and the cap.
Caspian Terns are a little bit bigger and have a heavier and more red bill than Royal Terns which have a thinner and more orange bill.
Caspian Terns also have a dark tip on the bill that Royal Terns don't have.
Non-breeding Royal Terns have extensive white on the crown, whereas Caspian Terns are mostly dark on the top of the head with a small area of white on the forehead.
Royal Terns have the full black cap only from March to June, while Caspian Terns have it much longer (February to October).

Together with a male Northern Pintail.

Khawr Rawri, Dhofar Region, Oman, 4 February 2017