The Tern

Nov 06, 2016 Features / Columnists, Interesting Creatures in Guyana

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Terns are seabirds in the family Sternidae that have a worldwide distribution and are normally found near the sea, rivers, or wetlands.
Previously considered a subfamily of the gulls, Laridae, they are now usually given full family status and divided into 11 genera. They are slender, lightly built birds with long, forked tails, narrow wings, long bills, and relatively short legs. Most species are pale grey above and white below, with a contrasting black cap to the head, but the marsh terns, the Inca tern, and some noddies have dark plumage for at least part of the year.
The sexes are identical in appearance, but young birds are readily distinguishable from adults. Terns have a non-breeding plumage, which usually involves a white forehead and much-reduced black cap.
The terns are birds of open habitats that typically breed in noisy colonies and lay their eggs on bare ground with little or no nest material. Marsh terns construct floating nests from the vegetation in their wetland habitats, and a few species build simple nests in trees, on cliffs or in crevices.
The white tern, uniquely, lays its single egg on a bare tree branch. Depending on the species, one to three eggs make up the clutch. Most species feed on fish caught by diving from flight, but the marsh terns are insect-eaters, and some large terns will supplement their diet with small land vertebrates. Many terns are long-distance migrants, and the Arctic tern may see more daylight in a year than any other animal.
Terns are long-lived birds and are relatively free from natural predators and parasites; most species are declining in numbers due directly or indirectly to human activities, including habitat loss, pollution, disturbance, and predation by introduced mammals.
The Chinese crested tern is in a critical situation and three other species are classed as endangered. International agreements provide a measure of protection, but adults and eggs of some species are still used for food in the tropics. The eggs of two species are eaten in the West Indies because they are believed to have aphrodisiac properties.
Terns range in size from the least tern, at 23 cm (9.1 in) in length and weighing 30–45 g (1.1–1.6 oz), to the Caspian tern at 48–56 cm (19–22 in), 500–700 g (18–25 oz). They are longer-billed, lighter-bodied, and more streamlined than gulls, and their long tails and long narrow wings give them an elegance in flight. Male and female plumages are identical, although the male can be two – five percent larger than the female and often has a relatively larger bill. Sea terns have deeply forked tails, and at least a shallow “V” is shown by all other species.
The noddies (genera Anous, Procelsterna and Gygis) have unusual notched-wedge shaped tails, the longest tail feathers being the middle-outer, rather than the central or outermost. Although their legs are short, terns can run well. They rarely swim, despite having webbed feet, usually landing on water only to bathe.
The majority of sea terns have light grey or white body plumage as adults, with a black cap to the head. The legs and bill are various combinations of red, orange, yellow, or black depending on species. The pale plumage is conspicuous from a distance at sea, and may attract other birds to a good feeding area for these fish-eating species.
When seen against the sky, the white underparts also help to hide the hunting bird from its intended prey. The Inca tern has mainly dark plumage, and three species that mainly eat insects, the black tern, white-winged tern, and black-bellied tern, have black underparts in the breeding season.
The Anous noddies have dark plumage with a pale head cap. The reason for their dark plumage is unknown, but it has been suggested that in tropical areas, where food resources are scarce, the less conspicuous colouration makes it harder for other noddies to detect a feeding bird.
Plumage type, especially the head pattern, is linked to the phylogeny of the terns, and the pale-capped, dark-bodied noddies are believed to have diverged earlier than the other genera from an ancestral white-headed gull, followed by the partially black-headed Onychoprion and Sternula groupings.
Juvenile terns typically have brown- or yellow-tinged upperparts, and the feathers have dark edges that give the plumage a scaly appearance. They have dark bands on the wings and short tails. In most species, the subsequent moult does not start until after migration, the plumage then becoming more like the adult, but with some retained juvenile feathers and a white forehead with only a partial dark cap.
Terns have a wide repertoire of vocalisations. For example, the common tern has a distinctive alarm, kee-yah, also used as a warning to intruders, and a shorter kyar, given as an individual takes flight in response to a more serious threat; this quietens the usually noisy colony while its residents assess the danger. Other calls include a down-slurred keeur given when an adult is approaching the nest with a fish, and a kip uttered during social contact.
Parents and chicks can locate one another by call, and siblings also recognise each other’s vocalisations from about the twelfth day after hatching, which helps to keep the brood together.
Vocal differences reinforce species separation between closely related birds such as the least and little terns, and can help humans distinguish similar species, such as common and Arctic terns, since flight calls are unique to each species.
Most terns hunt fish by diving, often hovering first, and the particular approach technique used can help to distinguish similar species at a distance. Sea terns often hunt in association with porpoises or predatory fish, such as bluefish, tuna or bonitos, since these large marine animals drive the prey to the surface.
Sooty terns feed at night as the fish rise to the surface, and are believed to sleep on the wing since they become waterlogged easily. Terns of several species will feed on invertebrates, following the plough or hunting on foot on mudflats.
The marsh terns normally catch insects in the air or pick them off the surface of fresh water. Other species will sometimes use these techniques if the opportunity arises. An individual tern’s foraging efficiency increases with its age.
The gull-billed tern is an opportunist predator, taking a wide variety of prey from marine, freshwater and terrestrial habitats. Depending on what is available it will eat small crabs, fish, crayfish, grasshoppers and other large insects, lizards and amphibians.
Warm-blooded prey includes mice and the eggs and chicks of other beach-breeding birds; least terns, little terns and members of its own species may be victims. The greater crested tern will also occasionally catch unusual vertebrate species such as agamid lizards and green sea turtle hatchlings, and follows trawlers for discards. (Source: Wikipedia)

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