Forster’s tern feeds primarily on small fish, arthropods and crustaceans (2) (3) (4) (5) (8). While foraging, Forster’s tern will typically fly back and forth over the water, with the bill pointing downwards and the feet folded against the body (2). Forster’s tern performs direct, shallow ‘plunge-dives’, generally only submerging the bill and front of the head when catching prey. Occasionally the bird will hover for several seconds above the surface before diving. Forster’s tern may also feed by flying low and skimming above the water, ‘dipping’ to pluck out prey, or it may hawk insects in the air while in flight. Forster’s tern may also sometimes forage from perches such as posts, bridges, telephone wires, or floating debris (2) (3) (4) (8).
This species breeds between May and mid-June, although breeding may begin as early as April on the Gulf Coast of the U.S. (3) (8). Generally, loose breeding colonies of around 5 to 250 pairs are formed, and individual pairs are thought to maintain small breeding territories around the nest (2) (3) (5). Both adults build the nest, which is typically positioned close to open water among floating and emergent vegetation. Most often the nest is little more than a cup constructed from plant material. Forster’s tern may also nest on mud or sand in a unlined or sparsely lined scrape in the ground, and, more rarely, on boards, spoil, fine shell and coarse gravel islands (2) (5) (8).
Forster’s tern usually lays 3 eggs, although clutch size may range from 2 to 5, and the eggs are incubated by both adults for around 23 to 25 days (3) (4). Like most tern species, Forster’s tern will fiercely defend its nest and young against intruders (4) (5). The young chicks are fed and guarded closely by the adults until they fledge at three to four weeks, after which both the adults and juveniles migrate from the breeding grounds to their wintering habitat (3) (4).