The common tern is an opportunistic predator, feeding predominantly on small fish, as well as crustaceans, insects and, occasionally, squid or other invertebrates (2) (5) (6) (8) (9) (11) (12) (13). Pairs of common terns frequently defend small feeding territories that often consist of linear strips of shoreline (2) (5).
Like most terns, this species catches its prey by plunge-diving. The common tern first locates its prey by hovering above the surface of the water, before swooping down and either picking food from the surface or diving below to catch its prey. The common tern may also attempt to steal food from other terns (2) (5) (6) (8) (9). It is a gregarious species, and shoals of fish often attract dense feeding flocks of over 1,000 individuals (2) (8).
Breeding occurs between April and June (8). Generally the common tern will breed in a large colony of up to several thousand pairs, although some pairs do nest alone (4) (5) (6) (8) (14). It typically arrives at established breeding sites and occupies a nesting territory around 15 to 25 days before egg laying begins (2). As part of the courtship ritual, the male common tern will bend forward and scrape a small hollow into the ground with the feet, followed by the female. The pair may make several different scrapes around the nest territory before the female selects one for laying (2). Courtship feeding also plays a major part in the breeding ritual, and the male will continually offer supplies of fish to the female prior to mating (15).
Both adult common terns contribute to the construction of the nest, which is typically a shallow depression, or a small mound, in open areas close to scattered vegetation and loose substrate, such as sand, soil, gravel or shell. The scrape is lined with vegetation and other debris which is gradually added to throughout the nesting period (2) (4) (5) (6) (8).
The common tern lays a clutch of between 1 and 4 eggs, (2) (4) (5) (6) (7), which are incubated by both the male and female for around 22 to 28 days (2) (4) (5) (6). Both adults feed the young chicks, which are capable of leaving the nest a few days after hatching (5) (6) (7) (14). The rate of nest failure is relatively high for the common tern, but if the first nest is lost a pair may re-lay after 8 to 12 days. Occasionally, the common tern may raise two broods in the same season, laying and incubating a second clutch of eggs while still feeding chicks from the first brood (2) (4) (6).
A highly migratory species, the common tern leaves the breeding grounds to travel to the wintering sites soon after the chicks fledge, typically between August and October (5).