Sandwich Tern

This is a medium-large tern, 36–41 cm (14–16 in) long with an 95–105 cm (37–41 in) wingspan, which is unlikely to be confused within most of its range. The weight ranges from 200–300 g; males average slightly heavier than females, but with much overlap. The thin, sharp bill is black with a yellow tip. Its short legs are black. Its upper wings are pale grey and its underparts white, and this tern looks very pale in flight, although the primary flight feathers darken during the summer. In autumn and winter, the adult's forehead becomes extensively white after the post-breeding moult in late summer (July-August); the black forehead is regained in the spring moult in February to March. Juvenile Sandwich terns have dark tips to their tails, an all-blackish bill (lacking the yellow tip, but sometimes yellowish at the base) and a scaly appearance on their back and wings, like juvenile roseate terns but with less black on the crown. Cabot's tern is the most similar species to Sandwich tern, sharing the black bill with a yellow tip, but differs structurally, with the bill being obviously stouter, and also differs in moult timing, losing its black forehead earlier in the summer. …

This species breeds in very dense colonies on coasts and islands, and exceptionally inland on suitable large freshwater lakes close to the coast. It nests in a ground scrape and lays one or two (rarely three) eggs. Unlike some of the smaller white terns, it is not very aggressive toward potential predators, relying on the sheer density of the nests, often only 20–30 cm (7.9–11.8 in) apart, and nesting close to other more aggressive species such as Arctic terns and black-headed gulls to avoid predation. Like all Thalasseus terns, the Sandwich tern feeds by plunge-diving for fish, almost invariably from the sea. It usually dives directly, and not from the "stepped-hover" favoured by Arctic terns. The offering of fish by the male to the female is part of the courtship display.

Migration. Sandwich tern is a long-distance migrant. Different breeding populations migrate to different wintering areas; those breeding in Great Britain migrate to the west coast of Africa from Senegal south to South Africa (a distance of over 10,000 km), while those breeding on the Caspian Sea coasts migrate to the Indian Subcontinent, south to Sri Lanka.