Common Tern

The common tern (Sterna hirundo) is a slender, long-winged tern breeding across temperate and subarctic parts of Europe, Asia and North America. Adults are about 31–35 cm long and weigh roughly 125 g on average. In breeding plumage they have pale grey upperparts, a black cap, a deeply forked tail and a red bill that usually has a dark tip.

The species is very similar to the Arctic tern (Sterna paradisaea). Common terns generally have a slightly heavier bill with a dark tip, longer legs, a somewhat shorter tail and whiter underparts. A darker wedge on the outer primaries often becomes more obvious through summer, although structure, flight style and calls are usually more reliable than any single plumage feature.

Common terns breed beside both salt and fresh water, including sheltered coasts, islands, estuaries, lakes, rivers and marshes. Nesting sites are usually open, sparsely vegetated surfaces such as gravel, sand or low rocky islands, although artificial rafts, rooftops and other structures may also be used where natural sites are scarce.

Feeding habitat is broader than nesting habitat. Birds range over coastal waters, lakes, rivers and wetlands wherever small fish are concentrated near the surface, so productive feeding areas close to a colony can be as important as the physical nesting site itself.

Small fish dominate the diet, supplemented by crustaceans, aquatic insects and other invertebrates. Breeding success can change quickly with prey availability because adults must repeatedly deliver small fish to growing chicks, so local water conditions, fish movements and weather can strongly affect breeding success from one season to the next.

Common terns hunt visually while flying over water. A bird may pause in a hover, dip the bill or head into the surface, or make a shallow plunge in which only part of the body enters the water. Their limited diving depth means that fish must be available in the upper part of the water column, and terns often exploit places where currents, turbulence or other predators bring small schooling fish closer to the surface.

The strategy is flexible. Although surface and shallow plunge-diving dominate when fish are available, common terns also catch flying insects and have occasionally been recorded taking other airborne prey or stealing food from other birds. This ability to switch between several capture methods helps the species exploit rapidly changing prey concentrations across both marine and freshwater habitats.

Common terns are long-distance migrants. Northern European birds leave from late summer into autumn and move mainly south along the Atlantic coasts of Europe and Africa, with many wintering well south of the equator before returning north in spring.

A 2025 tracking study of common terns breeding in southern Sweden followed six adults over nine migration journeys. All travelled to non-breeding areas south of the equator, mainly along the coasts from Angola to South Africa. Many paused around the North Sea or Britain and again along West Africa before reaching the main non-breeding range. Their migration therefore consisted of longer travel stages broken by stopovers.

Common terns are socially monogamous and usually breed in colonies or loose groups, often returning to familiar sites in successive years. Courtship includes aerial displays and fish presentation, with males offering prey to females. The nest itself is a shallow scrape, sometimes lined with small pieces of vegetation or shell.

A typical clutch contains 2–3 eggs. Both parents incubate for about three weeks and later defend and feed the chicks. The young are mobile soon after hatching but remain dependent on their parents for food and usually become capable of sustained flight at around 3–4 weeks of age. A study published in 2008 at a colony in northern Germany found that young remained around the colony for about 14–23 days after fledging in the two study years and continued gaining body mass before departure. Adults defend the colony vigorously, and neighbouring pairs may join together in driving away gulls, corvids and other potential predators.

The common tern has a very large global range and population and is classified as least concern on the IUCN Red List. Regional trends vary greatly, however, and some breeding populations have declined sharply despite the secure global category.

Common terns are strongly site-faithful and often concentrated at relatively few colonies, so local changes in nesting conditions or food supply can have large effects on regional numbers. Protecting established colonies and retaining suitable open nesting sites can therefore be important even where the species is not globally threatened.