Common tern

Appearance

The nominate subspecies of the common tern is 31–35 cm long, including a 6–9 cm fork in the tail, with a 77–98 cm wingspan.

It weighs 110–141 g. Breeding adults have pale grey upperparts, very pale grey underparts, a black cap, orange-red legs, and a narrow pointed bill that can be mostly red with a black tip, or all black, depending on the subspecies.

The common tern's upperwings are pale grey, but as the summer wears on, the dark feather shafts of the outer flight feathers become exposed, and a grey wedge appears on the wings. The rump and tail are white, and on a standing bird the long tail extends no further than the folded wingtips, unlike the Arctic and roseate terns in which the tail protrudes beyond the wings.

There are no significant differences between the sexes. In non-breeding adults the forehead and underparts become white, the bill is all black or black with a red base, and the legs are dark red or black. The upperwings have an obvious dark area at the front edge of the wing, the carpal bar. Terns that have not bred successfully may start moulting into non-breeding adult plumage from June, but late July is more typical, with the moult suspended during migration. There is also some geographical variation, Californian birds often being in non-breeding plumage during migration.

Juvenile common terns have pale grey upperwings with a dark carpal bar. The crown and nape are brown, and the forehead is ginger, wearing to white by autumn. The upperparts are ginger with brown and white scaling, and the tail lacks the adult's long outer feathers. Birds in their first post-juvenile plumage, which normally remain in their wintering areas, resemble the non-breeding adult, but have a duskier crown, dark carpal bar, and often very worn plumage. By their second year, most young terns are either indistinguishable from adults, or show only minor differences such as a darker bill or white forehead.

The common tern is an agile flyer, capable of rapid turns and swoops, hovering, and vertical take-off. When commuting with fish, it flies close to the surface in a strong head wind, but 10–30 m above the water in a following wind. Unless migrating, normally it stays below 100 m, and averages 30 km/h in the absence of a tail wind. Its average flight speed during the nocturnal migration flight is 43–54 km/h at a height of 1,000–3,000 m.

Distribution

Most populations of the common tern are strongly migratory, wintering south of their temperate and subarctic Northern Hemisphere breeding ranges. First summer birds usually remain in their wintering quarters, although a few return to breeding colonies some time after the arrival of the adults. In North America, the common tern breeds along the Atlantic coast from Labrador to North Carolina, and inland throughout much of Canada east of the Rocky Mountains. In the United States, some breeding populations can also be found in the states bordering the Great Lakes, and locally on the Gulf coast. There are small, only partially migratory, colonies in the Caribbean; these are in The Bahamas and Cuba, and off Venezuela in the Los Roques and Las Aves archipelagos.

New World birds winter along both coasts of Central and South America, to Argentina on the east coast and to northern Chile on the west coast. Records from South America and the Azores show that some birds may cross the Atlantic in both directions on their migration.

The common tern breeds across most of Europe, with the highest numbers in the north and east of the continent. There are small populations on the north African coast, and in the Azores, Canary Islands and Madeira. Most winter off western or southern Africa, birds from the south and west of Europe tending to stay north of the equator and other European birds moving further south. The breeding range continues across the temperate and taiga zones of Asia, with scattered outposts on the Persian Gulf and the coast of Iran. Small populations breed on islands off Sri Lanka, and in the Ladakh region of the Tibetan plateau. Western Asian birds winter in the northern Indian Ocean, and "S. h. tibetana" appears to be common off East Africa during the Northern Hemisphere winter. Birds from further north and east in Asia, such as "S. h. longipennis", move through Japan, Thailand and the western Pacific as far as southern Australia. There are small and erratic colonies in West Africa, in Nigeria and Guinea-Bissau, unusual in that they are within what is mainly a wintering area. Only a few common terns have been recorded in New Zealand, and this species' status in Polynesia is unclear. A bird ringed at the nest in Sweden was found dead on Stewart Island, New Zealand, five months later, having flown an estimated 25,000 km.

As long-distance migrants, common terns sometimes occur well outside their normal range. Stray birds have been found inland in Africa, and on the Maldives and Comoros islands; the nominate subspecies has reached Australia, the Andes, and the interior of South America. Asian "S. h. longipennis" has recent records from western Europe.

The common tern breeds over a wider range of habitats than any of its relatives, nesting from the taiga of Asia to tropical shores, and at altitudes up to 2,000 m in Armenia, and 4,800 m in Asia. It avoids areas which are frequently exposed to excessive rain or wind, and also icy waters, so it does not breed as far north as the Arctic tern. The common tern breeds close to freshwater or the sea on almost any open flat habitat, including sand or shingle beaches, firm dune areas, salt marsh, or, most commonly, islands. Flat grassland or heath, or even large flat rocks may be suitable in an island environment. In mixed colonies, common terns will tolerate somewhat longer ground vegetation than Arctic terns, but avoid the even taller growth acceptable to roseate terns; the relevant factor here is the different leg lengths of the three species. Common terns adapt readily to artificial floating rafts, and may even nest on flat factory roofs. Unusual nest sites include hay bales, a stump 0.6 m above the water, and floating logs or vegetation. There is a record of a common tern taking over a spotted sandpiper nest and laying its eggs with those of the wader. Outside the breeding season, all that is needed in terms of habitat is access to fishing areas, and somewhere to land. In addition to natural beaches and rocks, boats, buoys and piers are often used both as perches and night-time roosts.

Status

The common tern is classed as least concern on the IUCN Red List. It has a large population of 1,600,000–4,600,000 mature individuals and a huge breeding range estimated at 29,200,000 km2. Breeding numbers have been estimated at 250,000–500,000 pairs, the majority breeding in Asia. Up to 140,000 pairs breed in Europe. Fewer than 80,000 pairs breed in North America, with most breeding on the northeast Atlantic coast and a declining population of less than 10,000 pairs breeding in the Great Lakes region.

In the nineteenth century, the use of tern feathers and wings in the millinery trade was the main cause of large reductions in common tern populations in both Europe and North America, especially on the Atlantic coasts and inland. Sometimes entire stuffed birds were used to make hats. Numbers largely recovered early in the twentieth century mainly due to legislation and the work of conservation organizations. Although some Eurasian populations are stable, numbers in North America have fallen by more than 70% in the last 40 years, and there is an overall negative trend in the global estimates for this species.

Threats come from habitat loss through building, pollution or vegetation growth, or disturbance of breeding birds by humans, vehicles, boats or dogs. Local natural flooding may lead to nest losses, and some colonies are vulnerable to predation by rats and large gulls. Gulls also compete with terns for nest sites. Some birds are hunted in the Caribbean for commercial sale as food. Breeding success may be enhanced by the use of floating nest rafts, man-made islands or other artificial nest sites, and by preventing human disturbance. Overgrown vegetation may be burned to clear the ground, and gulls can be killed or discouraged by deliberate disturbance. Contamination with polychlorinated biphenyls resulted in enhanced levels of feminisation in male embryos, which seemed to disappear prior to fledging, with no effect on colony productivity, but dichlorodiphenyldichloroethylene, which results from the breakdown of DDT, led to very low levels of successful breeding in some US locations.

The common tern is one of the species to which the Agreement on the Conservation of African-Eurasian Migratory Waterbirds and the US-Canada Migratory Bird Treaty Act of 1918 apply. Parties to the AWEA agreement are required to engage in a wide range of conservation strategies described in a detailed action plan. The plan is intended to address key issues such as species and habitat conservation, management of human activities, research, education, and implementation. The North American legislation is similar, although there is a greater emphasis on protection.

Reproduction

Pairs are established or confirmed through aerial courtship displays in which a male and a female fly in wide circles up to 200 m or more, calling all the while, before the two birds descend together in zigzag glides. If the male is carrying a fish, he may attract the attention of other males too. On the ground, the male courts the female by circling her with his tail and neck raised, head pointing down, and wings partially open.

If she responds, they may both adopt a posture with the head pointed skywards. The male may tease a female with the fish, not parting with his offering until she has displayed to him sufficiently. Once courtship is complete, the male makes a shallow depression in the sand, and the female scratches in the same place. Several trials may take place until the pair settle on a site for the actual nest. The eggs may be laid on bare sand, gravel or soil, but a lining of debris or vegetation is often added if available, or the nest may be rimmed with seaweed, stones or shells.

The saucer-shaped scrape is typically 4 cm deep and 10 cm across, but may extend to as much as 24 cm wide including the surrounding decorative material. Breeding success in areas prone to flooding has been enhanced by the provision of artificial mats made from eelgrass, which encourage the terns to nest in higher, less vulnerable areas, since many prefer the mats to bare sand. The common tern tends to use more nest material than roseate or Arctic terns, although roseate often nests in areas with more growing vegetation.

Terns are expert at locating their nests in a large colony. Studies show that terns can find and excavate their eggs when they are buried, even if the nest material is removed and the sand smoothed over. They will find a nest placed 5 m from its original site, or even further if it is moved in several stages. Eggs are accepted if reshaped with plasticine or coloured yellow. This ability to locate the eggs is an adaptation to life in an unstable, wind-blown and tidal environment.

The peak time for egg production is early May, with some birds, particularly first-time breeders, laying later in the month or in June. The clutch size is normally three eggs; larger clutches probably result from two females laying in the same nest. Egg size averages 41 mm × 31 mm, although each successive egg in a clutch is slightly smaller than the first laid. The average egg weight is 20.2 g, of which 5% is shell. The egg weight depends on how well-fed the female is, as well as on its position in the clutch.

The eggs are cream, buff, or pale brown, marked with streaks, spots or blotches of black, brown or grey which help to camouflage them. Incubation is by both sexes, although more often by the female, and lasts 21–22 days, extending to 25 days if there are frequent disturbances at the colony which cause the adults to leave the eggs unattended; nocturnal predation may lead to incubation taking up to 34 days. On hot days the incubating parent may fly to water to wet its belly feathers before returning to the eggs, thus affording the eggs some cooling. Except when the colony suffers disaster, 90% of the eggs hatch. The precocial downy chick is yellowish with black or brown markings, and like the eggs, is similar to the equivalent stage of the Arctic tern. The chicks fledge in 22–28 days, usually 25–26. Fledged juveniles are fed at the nest for about five days, and then accompany the adults on fishing expeditions.

The young birds may receive supplementary feeds from the parents until the end of the breeding season, and beyond. Common terns have been recorded feeding their offspring on migration and in the wintering grounds, at least until the adults move further south in about December.

Like many terns, this species is very defensive of its nest and young, and will harass humans, dogs, muskrats and most diurnal birds, but unlike the more aggressive Arctic tern, it rarely hits the intruder, usually swerving off at the last moment. Adults can discriminate between individual humans, attacking familiar people more intensely than strangers....hieroglyph snipped... Nocturnal predators do not elicit similar attacks; colonies can be wiped out by rats, and adults desert the colony for up to eight hours when great horned owls are present.

Common terns usually breed once a year. Second clutches are possible if the first is lost. Rarely, a second clutch may be laid and incubated while some chicks from the first clutch are still being fed. The first breeding attempt is usually at four years of age, sometimes at three years. The average number of young per pair surviving to fledging can vary from zero in the event of the colony being flooded to over 2.5 in a good year. In North America, productivity was between 1.0 and 2.0 on islands, but less than 1.0 at coastal and inland sites. Birds become more successful at raising chicks with age. This continues throughout their breeding lives, but the biggest increase is in the first five years. The maximum documented lifespan in the wild is 23 years in North America and 33 years in Europe, but 12 years is a more typical lifespan.

Food

Like all "Sterna" terns, the common tern feeds by plunge-diving for fish, from a height of 1–6 m, either in the sea or in freshwater lakes and large rivers. The bird may submerge for a second or so, but to no more than 50 cm below the surface.

When seeking fish, this tern flies head-down and with its bill held vertically. It may circle or hover before diving, and then plunges directly into the water, whereas the Arctic tern favours a "stepped-hover" technique, and the roseate tern dives at speed from a greater height, and submerges for longer. The common tern typically forages up to 5–10 km away from the breeding colony, sometimes as far as 15 km.

It will follow schools of fish, and its west African migration route is affected by the location of huge shoals of sardines off the coast of Ghana; it will also track groups of predatory fish or dolphins, waiting for their prey to be driven to the sea's surface. Terns often feed in flocks, especially if food is plentiful, and the fishing success rate in a flock is typically about one-third higher than for individuals.

Terns have red oil droplets in the cone cells of the retinas of their eyes. This improves contrast and sharpens distance vision, especially in hazy conditions. Birds that have to see through an air/water interface, such as terns and gulls, have more strongly coloured carotenoid pigments in the cone oil drops than other avian species. The improved eyesight helps terns to locate shoals of fish, although it is uncertain whether they are sighting the phytoplankton on which the fish feed, or observing other terns diving for food. Tern's eyes are not particularly ultraviolet sensitive, an adaptation more suited to terrestrial feeders like the gulls.

The common tern preferentially hunts fish 5–15 cm long. The species caught depend on what is available, but if there is a choice, terns feeding several chicks will take larger prey than those with smaller broods. The proportion of fish fed to chicks may be as high as 95% in some areas, but invertebrate prey may form a significant part of the diet elsewhere. This may include worms, leeches, molluscs such as small squid, and crustaceans. In freshwater areas, large insects may be caught, such as beetles, cockchafers and moths. Adult insects may be caught in the air, and larvae picked from the ground or from the water surface. Prey is caught in the bill and either swallowed head-first, or carried back to the chicks. Occasionally, two or more small fish may be carried simultaneously. When adults take food back to the nest, they recognise their young by call, rather than visual identification.

The common tern may attempt to steal fish from Arctic terns, but might itself be harassed by kleptoparasitic skuas, laughing gulls, roseate terns, or by other common terns while bringing fish back to its nest. In one study, two males whose mates had died spent much time stealing food from neighbouring broods.

Terns normally drink in flight, usually taking seawater in preference to freshwater, if both are available. Chicks do not drink before fledging, reabsorbing water, and, like adults, excreting excess salt in a concentrated solution from a specialised nasal gland. Fish bones and the hard exoskeletons of crustaceans or insects are regurgitated as pellets. Adults fly off the nest to defecate, and even small chicks walk a short distance from the scrape to deposit their faeces. Adults attacking animals or humans will often defecate as they dive, often successfully fouling the intruder.

Predators

Rats will take tern eggs, and may even store large numbers in caches, and the American mink is an important predator of hatched chicks, both in North America, and in Scotland where it has been introduced. The red fox can also be a local problem. Because common terns nest on islands, the most common predators are normally other birds rather than mammals. The ruddy turnstone will take eggs from unattended nests, and gulls may take chicks. Great horned owls and short-eared owls will kill both adults and chicks, and black-crowned night herons will also eat small chicks. Merlins and peregrine falcons may attack flying terns; as with other birds, it seems likely that one advantage of flocking behaviour is to confuse fast-flying predators.

The common tern hosts feather lice, which are quite different from those found in Arctic terns, despite the close relationship of the two birds. It may also be infected by parasitic worms, such as the widespread "Diphyllobothrium" species, the duck parasite "Ligula intestinalis", and "Schistocephalus" species carried initially by fish. Tapeworms of the family Cyclophyllidea may also infect this species. The mite "Reighardia sternae" has been found in common terns from Italy, North America and China. A study of 75 breeding common terns found that none carried blood parasites. Colonies have been affected by avian cholera and ornithosis, and it is possible that the common tern may be threatened in the future by outbreaks of avian influenza to which it is susceptible. In 1961 the common tern was the first wild bird species identified as infected with avian influenza, the H5N3 variant being found in an outbreak of South African birds.