Sterna sandvicensis is the largest tern found regularly in the UK, with a body length of up to 43 cm and a wing span of 90 cm. The rump, underparts and back of the neck are mostly white, and the upperparts are pale grey. In breeding adults the forehead is black and the lores are white with a small black spot in front of the eye and black streaks underneath the eye. Dark outer primary feathers form a wedge on the upper wing. Black feathers at the back of the crown are elongated and raised, forming a shaggy crest which is raised when excited. The bill is black with a yellow tip. In summer the forehead, lores and crown are jet black. Juveniles have a dusky head, barred upperparts and black bill without the yellow tip.