The Caspian tern is the largest of all species of terns. With its 1 metre wingspan, it is similar in size to a black-backed gull. Caspian terns are silver-grey above and white below, with dark wing tips. The tail is relatively short and only slightly forked compared to other terns. The large bill is mostly bright red in adults, becoming dark near the tip, with the extreme tip  yellowish (only apparent at close range). Adults have black legs and a black cap to below the eye during the breeding season. The cap becomes speckled with white and less sharply delineated at other times of the year. Juveniles have some brown mottling on the back that is lost during the first autumn moult, while the diffuse brown cap is retained for longer. The bill is orange and smaller than adults at first. The legs and feet may be dull orange or black. The flight of Caspian terns is direct, with purposeful shallow beats.

Voice: a distinctive harsh ‘karh’ is often given in flight, particularly around breeding colonies.

Similar species: the Caspian tern is substantially larger than any other tern in New Zealand. The next largest species, the crested tern (a rare vagrant) is half the size by weight, and has a yellow bill. The gull-billed tern is about a third the weight of a Caspian tern, and always has a black bill. Caspian terns are sometimes mistaken for large gulls by inexperienced observers.

Caspain turn on rock