The Royal Tern (Thalasseus maximus) is the most common of fifteen terns in Costa Rica, regularly seen at Guanacaste beaches. In breeding plumage, the top of the Royal Tern's head, including the eyes, is black, including the eyes, is black, the bill is bright orange, and a deeply forked tail.

The Royal Tern feeds by flying over shallow coastline waters and estuaries in search of small fish, squid, shrimp and crabs. From a hight of 5-10 meter (15-30 feet) above the water, the tern plunge-dives to capture its prey but does not submerge. It may also skim the surface and dip its head to capture fish.

The Royal Tern makes its nest scrape on the ground on low-lying islands. The pair defecates directly on the nest rim, perhaps to reinforce the nest against flooding. After a few weeks, the nest rim hardens.

Young Royal Terns leave the nest scrape within one day after hatching and congregate together in a group known as a crèche. Eventually all of the chicks in a colony come to the crèche, which can have thousands of chicks ranging in age from two to 35 days old. A pair of Royal Terns will feed only their own chick, and manage to find it in the crowd, probably by recognizing its call.