Caspian tern  (Sterna caspia)

The Caspian tern is classified as Least Concern (LC) on the IUCN Red List (1).

The largest of all terns (3), the Caspian tern (Sterna caspia) is a majestic seabird recognised by its stout, blood red, black tipped bill, and black head cap (2). A largely grey and white bird except for black primary feathers, the Caspian tern also has dark brown eyes and black legs (2). Outside of the breeding season the normally bold black cap becomes flecked with white. There are two colour variations of juvenile Caspian terns: creamy-white with yellow legs and orange feet, and greyish-brown with dark legs and olive feet (2).

The Caspian tern has a very large range that includes coastlines and inland lakes of North America, Central America, the northern regions of South America, Africa, northern Europe and the Middle East, through to Asia, including the inland seas of central Asia and Russia (4).

The Caspian tern is found in a variety of coastal and inland habitats including coastal lagoons, salt marshes, estuaries, coastal inshore waters, continental shelf waters, bogs and marshes, freshwater lakes, water storage areas and inland seas (5). However, it is rarely found over open water at sea (2).

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).

The Caspian tern is primarily threatened by the loss and degradation of its breeding habitat due to the increased growth of shrubby vegetation, which is thought to be caused by the introduction of exotic plant species. This threat may be exacerbated in the future as rising sea levels caused by global climate change could destroy many breeding sites. The Caspian tern is also threatened by disturbance during the breeding season, which can cause breeding birds to abandon their eggs (2) (3), while contamination from toxic pollutants can also affect breeding success by reducing egg shell thickness. In addition, the Caspian tern is susceptible to avian botulism and may become threatened by future outbreaks of this fatal disease (1) (4) (6).

Plans to conserve the Caspian tern include habitat and vegetation management, the use of artificial nests, predator management, and minimisation of disturbance. The creation of artificial islands, with submerged vegetation and fish spawning habitats, has also benefited the species (1) (4). Three such islands were constructed in Hamilton Harbour, Lake Ontario in 1995 to 1996, and in subsequent years has produced between 200 and 300 clutches (3). The Caspian tern also occurs in several National Parks (4).

View information on this species at the UNEP World Conservation Monitoring Centre.

Environment Agency - Abu Dhabi is a principal sponsor of ARKive. EAD is working to protect and conserve the environment as well as promoting sustainable development in the Emirate of Abu Dhabi.

For more information on the conservation of birds, see:

• The Royal Society for the Protection of Birds:
  http://www.rspb.org.uk/
• The American Bird Conservancy:
  http://www.abcbirds.org/

 For more information on the Caspian tern and other bird species, see:

• BirdLife International:
  http://www.birdlife.org/

 For more information about conservation in the Emirates, see:

• The Environment Agency – Abu Dhabi:
  http://www.ead.ae/en