The Caspian tern will breed either in large colonies of terns or other bird species between April and June in the Northern Hemisphere, and September to December in the Southern Hemisphere, apart from in Northern Australia where it may breed all year round (2). The Caspian tern is typically monogamous (3), with the male bird displaying with bowing movements of the head and offerings of fish to attract a female mate. To reinforce the mating bond, pairs from previous seasons perform a display flight, climbing to 200 metres in the air, before descending to the ground, where the male offers the female food. The nest is a simple depression made in sand, gravel, a layer of shells, ground vegetation or dried mud (2) (4). A clutch of one to three eggs is laid over a few days and incubated by both parent birds over a period of approximately 27 days (2) (5). The Caspian tern cares for its offspring for a longer period than any other tern species, with some juveniles remaining partly dependent on the parent birds for several months. Sexual maturity is reached at three years of age (3).
The diet of the Caspian tern consists mainly of small to medium-sized fish, between 5 and 25 centimetres long, but may also include eggs and chicks of other birds, as well as carrion (2) (4) (5). The Caspian tern catches its food by flying slowly or hovering briefly above the water before plunging down to catch the prey in its serrated bill. It may also forage around groups of predatory fish that drive schools of smaller fish towards the water’s surface, allowing the Caspian tern to snatch prey by just submerging its head (2) (4) (5).
Due to its larger size, the Caspian tern is more awkward on land than other terns, and tends to walk in an ungainly waddle. It is, however, more graceful in flight, flying with deep powerful wing-beats, producing a strong and swift motion. The Caspian tern is active both day and night, although most activity occurs during the day (3).