The Caspian tern (Hydroprogne caspia) is the largest tern species in the world. It is a thick-billed bird roughly the size of the common gull. Its call is a harsh squawk.
The breeding population of Helsinki consists of only a few pairs of Caspian terns. The majority of the Caspian terns seen in Helsinki come here to fish, flying dozens of kilometres from the archipelagos of Porvoo and Kirkkonummi, where they form large nesting colonies. The Caspian tern is a cosmopolitan bird that nests on all the continents, including Australia.