A particularly gregarious species, the sandwich tern often forms feeding flocks where prey is abundant or concentrated (8). It feeds almost entirely on small fish, squid and crustaceans (2) (4), which it catches by plunging into the water from a height of five to seven metres (2). When foraging over shallow water, the sandwich tern may hover stationary over the water while it searches for prey, before ascending and rapidly diving to feed. It may also snatch its prey from the surface of the water, by flying low and skimming the waves (2). The sandwich tern has been recorded feeding over fishing nets, close to sea lions and over porpoise pods, where it scavenges small fish (4).
The sandwich tern usually breeds in dense colonies with other terns, such as the royal tern (Sterna maxima), the roseate tern (Sterna dougallii), the common tern (Sterna hirundo) and the Arctic tern (Sterna paradisaea). Colonies are also typically found in association with other seabirds, including the black-headed gull (Larus ridibundus), the laughing gull (Larus atricilla), and the black skimmer (Rynchops niger) (2) (8). The timing of breeding is variable with location, but is typically around April to May in the U.S. and the Caribbean, May and June in Europe, June in Brazil and December in Argentina (2) (4).
Generally, the nest is a shallow scrape which is excavated by both adults (2) (8). The male and female sandwich tern walk together to potential nest sites, alternately making shallow scrapes by planting the breast on the substrate and kicking away the sand or gravel behind with the feet (2). The chosen nest is gradually lined with shell, excrement, vegetation or other materials throughout the breeding season (2) (4).
The female sandwich tern lays a clutch of 1 or 2 eggs, which are incubated for around 21 to 29 days. Both adults share the incubation duties equally, except for in the first few days after laying when the female is fed at the nest site by the male (2) (4). Once hatched, the young may gather together in a group, called a ‘crèche’, which is attended by one or several adults, and remain there until fledging at around 27 to 29 days. In general, sandwich tern chicks are whitish, buff, grey or brown, with many black speckles (4). The chicks are fed by both adults, and continue to be fed after fledging by one of the adult sandwich terns, probably the female (2).