Caspian tern

The Caspian tern (Hydroprogne caspia or Sterna caspia) is widely distributed around the world, being found on every continent except Antarctica. Its numbers appear to be increasing across much of its range, so it is fortunate to be regarded as a species of “least concern” by BirdLife International.

Depending on its location it is either sedentary or migratory. North American birds migrate between the breeding grounds of the Great Lakes and northern coasts to the southern coasts and the Caribbean, while in Europe the movement is between the coasts of the Baltic and Black Seas and tropical Africa.


Appearance

The Caspian tern is the largest species of tern, having an adult body length of 48-56 centimetres (19-22 inches) and a wingspan of 127-140 centimetres (4-4.5 feet), which is greater than that of the common gull.

The body is white with the back and wings being pale grey. The cap of the typically tern-like flat head is black but the Caspian tern is particularly noticeable from its large bright red bill with a dark tip. The legs and feet are black. The tail has a shallow fork. In juvenile birds the plumage is greyer, the bill more orange than red and the legs paler. Male and female Caspian terns are similar to each other in appearance.


Habitat

The Caspian tern is generally a coastal species, preferring sheltered sea coasts, estuaries, harbours and the like. During the breeding season it prefers sandy or shingle beaches or sparsely vegetated rocky environments, often on remote offshore islands. It is only likely to be seen inland during migration.


Behaviour and Feeding

Outside the breeding season the Caspian tern is not highly gregarious, although it can form flocks during migration. Groups of Caspian terns may sometimes be seen feeding together at places where food is plentiful, or at night roosts.

Food is mainly medium-sized fish (such as herring), although invertebrates and the eggs and young of other bird species are sometimes taken. Caspian terns are unusual among tern species in that they will also eat carrion.

The flight of the Caspian tern is not unlike that of gulls, with slow ponderous wingbeats. When feeding they will fly not far above the water, hover for a few seconds, then dive to take a fish.

The voice of the Caspian tern is a deep hoarse “kree-ark”. Young birds will call with a squeaky “wee-vee”.


Breeding

Caspian terns return to their breeding grounds in April or May (assuming a Northern Hemisphere population). Breeding colonies can be large, but they are also found breeding as lone pairs or scattered among colonies of other species.

The nest is a scrape in the ground with only a few pieces of plant matter. The usual clutch comprises two eggs, laid in May or June, which both partners incubate for 20-22 days. Shortly after hatching, the chicks disperse to hide in neighbouring vegetation, with food being brought to them by their parents. Caspian terns have been noted to fly up to 40 miles from the nest site on foraging trips.

The young are able to fly and fend for themselves at about 30-35 days after hatching.


Conservation

As noted above, the Caspian tern is under no overall threat globally, although this is not always true of local populations. Threats come mainly from habitat loss due to human activity, with rising sea levels being a concern in some areas. The use of fertilisers and pesticides, the residues of which can poison fish, can affect the breeding ability of birds which feed on those fish, the Caspian tern included.

© John Welford