An opportunistic feeder, the royal tern feeds predominantly on small fish, as well as squid and crustaceans (2) (8) (9) (12). It may also follow fishing boats, feeding on discarded bycatch (3). Generally, the royal tern forages alone or in pairs, but may aggregate in large flocks where prey is abundant (2) (3) (12), and especially when schools of predatory fish chase smaller fish towards the surface (12). It typically remains close to the shore during foraging trips, but may fly considerably further from the colony when searching for food to feed the young (3). The royal tern searches for its prey by flying low over the surface of the water and, once it has located suitable prey, it will plunge-dive vertically downwards, seizing the prey with its bill and swallowing it whole (2) (3).
North American and African populations of the royal tern typically breed between April and August (9) (11), and the South American population between October and March (11). It nests in large, dense colonies of several thousand pairs (8) (12), often in association with other species such as the laughing gull (Larus atricilla), the Caspian tern (Sterna caspia) and the sandwich tern (Sterna sandvicensis) (3) (9) (11).
Pairs perform courtship displays close to potential nest sites, often consisting of complex aerial displays, with much soaring and manoeuvring (3) (12) (13). The male royal tern will also approach the female on the ground, circling, strutting and raising its crest before offering food (14). Once pair bonds have been established, both the male and the female will select a nest site, circling the chosen area several times (2) (3) (14).
The nest is a simple, shallow scrape in the ground. Both members of the pair contribute to building the nest, taking it in turns to scrape away the ground with the feet, before sitting and swivelling the body to round out the nest (2). The nest is typically positioned fairly close to the high tide line, on areas of open, bare ground with little or no vegetation (2) (9) (14). The royal tern defecates directly onto the rim of the nest to harden the edges, to give it some protection against flooding (2) (8).
Colonies of royal terns have very high densities of nests, which become very tightly packed during the breeding season. The nesting pair defends a small territory directly around the nest, which extends to the reach of the incubating adult (12). The royal tern is fiercely defensive of its nest and young (5). A single egg is laid each breeding season, and both the male and the female royal tern share the incubation duties for 30 to 31 days (2) (3) (12).
Shortly after hatching, the royal tern chick leaves the nest and gathers together with other chicks, forming large crèches (2) (3) (8) (12) (13). This may also contain chicks of other species, such as the sandwich tern (2). The adult royal terns recognise their young by its call, and feed only their own chick in the crèche. The chick remains in the crèche until it fledges at 28 to 35 days old, during which time it is fed and brooded by both adults (2) (8) (13). Royal tern chicks have an extremely long period of post-fledging parental care (15), remaining dependent on the adult terns for feeding and protection for five to eight months after hatching (2) (3) (12).