Common tern

The common tern[2] (Sterna hirundo) is a seabird in the family Laridae. This bird has a circumpolar distribution, its four subspecies breeding in temperate and subarctic regions of Europe, Asia and North America. It is strongly migratory, wintering in coastal tropical and subtropical regions. Breeding adults have light grey upperparts, white to very light grey underparts, a black cap, orange-red legs, and a narrow pointed bill. Depending on the subspecies, the bill may be mostly red with a black tip or all black. There are a number of similar species, including the partly sympatric Arctic tern, which can be separated on plumage details, leg and bill colour, or vocalisations.

Breeding in a wider range of habitats than any of its relatives, the common tern nests on any flat, poorly vegetated surface close to water, including beaches and islands, and it readily adapts to artificial substrates such as floating rafts. The nest may be a bare scrape in sand or gravel, but it is often lined or edged with whatever debris is available. Up to three eggs may be laid, their dull colours and blotchy patterns providing camouflage on the open beach. Incubation is by both sexes, and the eggs hatch in around 21–22 days, longer if the colony is disturbed by predators. The downy chicks fledge in 22–28 days. Like most terns, this species feeds by plunge-diving for fish, either in the sea or in freshwater, but molluscs, crustaceans and other invertebrate prey may form a significant part of the diet in some areas.

Eggs and young are vulnerable to predation by mammals such as rats and American mink, and large birds including gulls, owls and herons. Common terns may be infected by lice, parasitic worms, and mites, although blood parasites appear to be rare. Its large population and huge breeding range mean that this species is classed as being of least concern, although numbers in North America have declined sharply in recent decades. Despite international legislation protecting the common tern, in some areas populations are threatened by habitat loss, pollution or the disturbance of breeding colonies.

Common-tern,web
Twisted head
Common tern
Common tern with fish
Scientific classification
Kingdom: Animalia
Phylum: Chordata
Class: Aves
Order: Charadriiformes
Family: Laridae
Genus: Sterna
Species:
S. hirundo
Binomial name
Sterna hirundo
Map showing the breeding range of Sterna hirundo (most of temperate northern hemisphere), and wintering areas (coasts in tropics and southern hemisphere).
     Breeding      Resident      Non-breeding      Passage      Vagrant (seasonality uncertain)
Synonyms

Sterna fluviatilis Naumann, 1839

Taxonomy

The terns are small to medium-sized seabirds closely related to the gulls, skimmers and skuas. They are gull-like in appearance, but typically have a lighter build, long pointed wings (which give them a fast, buoyant flight), a deeply forked tail, slender legs,[3] and webbed feet.[4] Most species are grey above and white below, and have a black cap which is reduced or flecked with white in the non-breeding season.[3]

The common tern's closest relatives appear to be the Antarctic tern,[5] followed by the Eurasian Arctic and roseate terns. Genetic evidence suggests that the common tern may have diverged from an ancestral stock earlier than its relatives.[6] No fossils are known from North America, and those claimed in Europe are of uncertain age and species.[5]

The common tern was first described by Linnaeus in his Systema Naturae in 1758 under its current scientific name, Sterna hirundo.[7] The word "stearn" was used in Old English and a similar word was used by the Frisians for tern.[8] "Stearn" appears in the poem The Seafarer, written around 1000 AD.[8] Linnaeus adopted this word for the genus name Sterna. The Latin for swallow is "hirundo" and refers here to the tern's superficial likeness to that unrelated bird, which has a similar light build and long forked tail.[9] This resemblance also leads to the informal name "sea swallow",[10] recorded from at least the seventeenth century.[9] The Scots names picktarnie,[11] tarrock[12] and their many variants are also believed to be onomatopoeic, derived from the distinctive call.[9] Due to the difficulty in distinguishing the two species, all the informal common names are shared with the Arctic tern.[13]

Four subspecies of the common tern are generally recognized, although S. h. minussensis is sometimes considered to be an intergrade between S. h. hirundo and S. h. longipennis.[14][15]

Subspecies Breeding range Distinctive features
S. h. hirundo
Linnaeus, 1758
Europe, North Africa, Asia east to western Siberia and Kazakhstan, and North America.[16] Differences between the American and Eurasian populations are minimal. American birds have a slightly shorter wing length on average, and the extent of the black tip on the upper mandible tends to be less than in birds from Scandinavia and further east in Eurasia. The proportion of black on the bill is at its minimum in the west of Europe, so British breeders are very similar to American birds in this respect.[16]
S. h. minussensis
Sushkin, 1925
Lake Baikal east to northern Mongolia and southern Tibet.[17] Paler upper body and wings than S. h. longipennis, black-tipped crimson bill.[17]
S. h. longipennis
Nordmann, 1835
Central Siberia to China, also Alaska.[16] Darker grey than the nominate subspecies, with shorter black bill, darker red-brown legs, and longer wings.[16]
S. h. tibetana
Saunders, 1876
Himalayas to southern Mongolia and China.[16] Like the nominate subspecies, but bill is shorter with broader black tip.[16]

Description

Sterna hirundo in Finland
Adult S. h. hirundo in the harbour of Jyväskylä, Finland
Sterna hirundo -Nantucket National Wildlife Refuge, Massachusetts, USA -head-8
Adult S. h. hirundo in breeding plumage at Nantucket National Wildlife Refuge, Massachusetts

The nominate subspecies of the common tern is 31–35 cm (12–14 in) long, including a 6–9 cm (2.4–3.5 in) fork in the tail, with a 77–98 cm (30–39 in) wingspan. It weighs 110–141 g (3.9–5.0 oz).[16] 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.[18] 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.[19] 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.[19] 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.[16]

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.[16] 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.[20]

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 (33–98 ft) above the water in a following wind. Unless migrating, normally it stays below 100 m (330 ft), and averages 30 km/h (19 mph) in the absence of a tail wind.[5] Its average flight speed during the nocturnal migration flight is 43–54 km/h (27–34 mph)[21] at a height of 1,000–3,000 m (3,300–9,800 ft).[5]

Moult

Commonternprimary
Detail of primary feather

The juvenile starts moulting into adult plumage in its first October; the head, tail and body plumage is replaced first, mostly by February, then the wing feathers. The primaries are replaced in stages; the innermost feathers moult first, then replacement is suspended during the southern winter (birds of this age staying in their wintering areas) and recommences in the autumn. In May to June of the second year a similar moult sequence starts, with a pause during primary moult for birds that return north, but not for those that stay in the winter quarters. A major moult to adult breeding plumage occurs in the next February to June, between 40–90% of feathers being replaced.[16] Old primary feathers wear away to reveal the blackish barbs beneath. The moult pattern means that the oldest feathers are those nearest the middle of the wing, so as the northern summer progresses, a dark wedge appears on the wing due to this feather ageing process.[18]

Terns are unusual in the frequency in which they moult their primaries, which are replaced at least twice, occasionally three times in a year. The visible difference in feather age is accentuated in the greater ultraviolet reflectance of new primaries, and the freshness of the wing feathers is used by females in mate selection.[22] Experienced females tend to accept mates which best show their fitness through the quality of their wing feathers.[23] Rarely, a very early moult at the nesting colony may be linked to breeding failure, both the onset of moult and reproductive behaviour being linked to falling levels of the hormone prolactin.[24]

Similar species

Arctic Tern (Sterna paradisaea), Ward of Clugan - geograph.org.uk - 1357999
This Arctic tern shows the all-red bill, greyer underparts, translucent flight feathers, and narrow black border to the wings which separate it from the common tern.

There are several terns of a similar size and general appearance to the common tern. A traditionally difficult species to separate is the Arctic tern, and until the key characteristics were clarified, distant or flying birds of the two species were often jointly recorded as "commic terns". Although similar in size, the two terns differ in structure and flight. The common tern has a larger head, thicker neck, longer legs, and more triangular and stiffer wings than its relative, and has a more powerful, direct flight.[25] The Arctic tern has greyer underparts than the common, which make its white cheeks more obvious, whereas the rump of the common tern can be greyish in non-breeding plumage, compared to the white of its relative. The common tern develops a dark wedge on the wings as the breeding season progresses, but the wings of Arctic stay white throughout the northern summer. All the flight feathers of the Arctic tern are translucent against a bright sky, only the four innermost wing feathers of the common tern share this property.[25][26] The trailing edge of the outer flight feathers is a thin black line in the Arctic tern, but thicker and less defined in the common.[19] The bill of an adult common tern is orange-red with a black tip, except in black-billed S. h. longipennis, and its legs are bright red, while both features are a darker red colour in the Arctic tern, which also lacks the black bill tip.[25]

In the breeding areas, the roseate tern can be distinguished by its pale plumage, long, mainly black bill and very long tail feathers.[26] The non-breeding plumage of roseate is pale above and white, sometimes pink-tinged, below. It retains the long tail streamers, and has a black bill.[27] In flight, the roseate's heavier head and neck, long bill and faster, stiffer wingbeats are also characteristic.[28] It feeds further out to sea than the common tern.[27] In North America, the Forster's tern in breeding plumage is obviously larger than the common, with relatively short wings, a heavy head and thick bill, and long, strong legs; in all non-breeding plumages, its white head and dark eye patch make the American species unmistakable.[29]

In the wintering regions, there are also confusion species, including the Antarctic tern of the southern oceans, the South American tern, the Australasian white-fronted tern and the white-cheeked tern of the Indian Ocean. Identification may be aided by the plumage differences due to "opposite" breeding seasons. The Antarctic tern is more sturdy than the common, with a heavier bill. In breeding condition, its dusky underparts and full black cap outline a white cheek stripe. In non-breeding plumages, it lacks, or has only an indistinct, carpal bar, and young birds show dark bars on the tertials, obvious on the closed wing and in flight.[30][31] The South American tern is larger than the common, with a larger, more curved red bill, and has a smoother, more extensive black cap in non-breeding plumage.[32] Like Antarctic, it lacks a strong carpal bar in non-breeding plumages, and it also shares the distinctive barring of the tertials in young birds.[33] The white-fronted tern has a white forehead in breeding plumage, a heavier bill, and in non-breeding plumage is paler below than the common, with white underwings.[34] The white-cheeked tern is smaller, has uniform grey upperparts, and in breeding pumage is darker above with whiter cheeks.[35]

Juvenile common terns are easily separated from similar-aged birds of related species. They show extensive ginger colouration to the back, and have a pale base to the bill. Young Arctic terns have a grey back and black bill, and juvenile roseate terns have a distinctive scalloped "saddle".[19] Hybrids between common and roseate terns have been recorded, particularly from the US, and the intermediate plumage and calls shown by these birds is a potential identification pitfall. Such birds may have more extensive black on the bill, but confirmation of mixed breeding may depend on the exact details of individual flight feathers.[16]

Voice

The common tern has a wide repertoire of calls, which have a lower pitch than the equivalent calls of Arctic terns. The most distinctive sound is the alarm KEE-yah, stressed on the first syllable, in contrast to the second-syllable stress of the Arctic tern. The alarm call doubles up as a warning to intruders, although serious threats evoke a kyar, given as a tern takes flight, and quietens the usually noisy colony while its residents assess the danger.[36] A down-slurred keeur is given when an adult is approaching the nest while carrying a fish, and is possibly used for individual recognition (chicks emerge from hiding when they hear their parents giving this call). Another common call is a kip uttered during social contact. Other vocalizations include a kakakakaka when attacking intruders, and a staccato kek-kek-kek from fighting males.[36]

Parents and chicks can locate one another by call, and siblings also recognise each other's vocalisations from about the twelfth day from hatching, which helps to keep the brood together.[37][38]

Distribution and habitat

Sterna hirundo - Boat Harbour
Non-breeding adult in Australia

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.[20] 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.[39] There are small, only partially migratory, colonies in the Caribbean; these are in The Bahamas and Cuba,[40] and off Venezuela in the Los Roques and Las Aves archipelagos.[41]

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.[20][39] Records from South America and the Azores show that some birds may cross the Atlantic in both directions on their migration.[42][43]

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.[44] The breeding range continues across the temperate and taiga zones of Asia, with scattered outposts on the Persian Gulf and the coast of Iran.[45] Small populations breed on islands off Sri Lanka,[46][47] and in the Ladakh region of the Tibetan plateau.[48] Western Asian birds winter in the northern Indian Ocean,[20][49] and S. h. tibetana appears to be common off East Africa during the northern hemisphere winter.[50] 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.[20] 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.[45] Only a few common terns have been recorded in New Zealand,[51] and this species' status in Polynesia is unclear.[52] 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 (15,000 mi).[53]

As long distance migrants, common terns sometimes occur well outside their normal range. Stray birds have been found inland in Africa (Zambia and Malawi), and on the Maldives and Comoros islands;[54] the nominate subspecies has reached Australia,[34] the Andes, and the interior of South America.[32][55] Asian S. h. longipennis has recent records from western Europe.[56]

The common tern breeds over a wider range of habitats than any of its relatives, nesting from the taiga of Asia to tropical shores,[57] and at altitudes up to 2,000 m (6,600 ft) in Armenia, and 4,800 m (15,700 ft) in Asia.[44] 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.[57] 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.[58] Common terns adapt readily to artificial floating rafts, and may even nest on flat factory roofs.[57] Unusual nest sites include hay bales, a stump 0.6 m (2 ft) 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.[59] 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.[57]

Behaviour

Territory

Sterna hirundo -nest with three eggs-8
Three eggs in a nest on Great Gull Island
CommonTern-Chick
A chick on an island off the coast of Maine

The common tern breeds in colonies which do not normally exceed 2,000 pairs,[44] but may occasionally number more than 20,000 pairs.[60] Colonies inland tend to be smaller than on the coast. Common terns often nest alongside other coastal species, such as Arctic,[61] roseate and Sandwich terns, black-headed gulls,[62][63] and black skimmers.[64] Especially in the early part of the breeding season, for no known reason, most or all of the terns will fly in silence low and fast out to sea. This phenomenon is called a "dread".[44]

Sterna hirundo -Massachusetts, USA -juvenile-8
This autumn juvenile in Massachusetts has a white forehead, having lost the ginger colouration characteristic of younger birds

On their return to the breeding sites, the terns may loiter for a few days before settling into a territory,[65] and the actual start of nesting may be linked to a high availability of fish.[66] Terns defend only a small area, with distances between nests sometimes being as little as 50 cm (20 in), although 150–350 cm (59–138 in) is more typical. As with many birds, the same site is re-used year after year, with a record of one pair returning for 17 successive breeding seasons. Around 90% of experienced birds reuse their former territory, so young birds must nest on the periphery, find a bereaved mate, or move to another colony.[65] A male selects a nesting territory a few days after his arrival in the spring, and is joined by his previous partner unless she is more than five days late, in which case the pair may separate.[67]

The defence of the territory is mainly by the male, who repels intruders of either sex. He gives an alarm call, opens his wings, raises his tail and bows his head to show the black cap. If the intruder persists, the male stops calling and fights by bill grappling until the intruder submits by raising its head to expose the throat. Aerial trespassers are simply attacked, sometimes following a joint upward spiralling flight.[65] Despite the aggression shown to adults, wandering chicks are usually tolerated, whereas in a gull colony they would be attacked and killed. The nest is defended until the chicks have fledged, and all the adults in the colony will collectively repel potential predators.[68]

Breeding

Pairs are established or confirmed through aerial courtship displays in which a male and a female fly in wide circles up to 200 m (660 ft) 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.[69] 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.[69] The eggs may be laid on bare sand, gravel or soil, but a lining of debris or vegetation is often added if available,[44] or the nest may be rimmed with seaweed, stones or shells. The saucer-shaped scrape is typically 4 cm (1.6 in) deep and 10 cm (3.9 in) across, but may extend to as much as 24 cm (9.4 in) wide including the surrounding decorative material.[70] 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.[71] The common tern tends to use more nest material than roseate or Arctic terns, although roseate often nests in areas with more growing vegetation.[72][73]

Sterna hirundo -hovering to protect nest-8
Hovering and screaming to deter intruders on Great Gull Island
Sterna hirundo MWNH 0472
Egg, Collection Museum Wiesbaden

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 (16 ft) from its original site, or even further if it is moved in several stages. Eggs are accepted if reshaped with plasticine or coloured yellow (but not red or blue). This ability to locate the eggs is an adaptation to life in an unstable, wind-blown and tidal environment.[58]

The peak time for egg production is early May, with some birds, particularly first-time breeders, laying later in the month or in June.[59][70] 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 (1.6 in × 1.2 in), although each successive egg in a clutch is slightly smaller than the first laid.[70] The average egg weight is 20.2 g (0.71 oz), of which 5% is shell.[74] 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.[70] Incubation is by both sexes, although more often by the female, and lasts 21–22 days,[74] extending to 25 days if there are frequent disturbances at the colony which cause the adults to leave the eggs unattended;[70] nocturnal predation may lead to incubation taking up to 34 days.[59] 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.[5] Except when the colony suffers disaster, 90% of the eggs hatch.[75] The precocial downy chick is yellowish with black or brown markings,[70] and like the eggs, is similar to the equivalent stage of the Arctic tern.[76] The chicks fledge in 22–28 days,[74] usually 25–26.[44] 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.[5][77]

Batalla de golondrinas de mar (Sterna hirundo)

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.[78] Nocturnal predators do not elicit similar attacks;[79] colonies can be wiped out by rats, and adults desert the colony for up to eight hours when great horned owls are present.[80]

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.[81] 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.[5][76] The maximum documented lifespan in the wild is 23 years in North America[82][83] and 33 years in Europe,[84][85] but 12 years is a more typical lifespan.[74]

Food and feeding

Sterna hirundo -West Bromwich, England -flying-8
Flying over a pond in England. The head and bill point down during a search for fish.

Like all Sterna terns, the common tern feeds by plunge-diving for fish, from a height of 1–6 m (3.3–19.7 ft), 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 (20 in) below the surface.[86] When seeking fish, this tern flies head-down and with its bill held vertically.[58] It may circle or hover before diving, and then plunges directly into the water, whereas the Arctic tern favours a "stepped-hover" technique,[87] and the roseate tern dives at speed from a greater height, and submerges for longer.[88] The common tern typically forages up to 5–10 km (3.1–6.2 mi) away from the breeding colony, sometimes as far as 15 km (9.3 mi).[89] 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;[86] it will also track groups of predatory fish or dolphins, waiting for their prey to be driven to the sea's surface.[89][90] 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.[86]

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.[91] 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.[92] 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.[93] Tern's eyes are not particularly ultraviolet sensitive, an adaptation more suited to terrestrial feeders like the gulls.[94]

Sterna hirundo -Nantucket National Wildlife Refuge, Massachusetts, USA -adult and juvenile-8
An adult bringing a sand eel to a juvenile at Nantucket National Wildlife Refuge

The common tern preferentially hunts fish 5–15 cm (2.0–5.9 in) long.[59][86] 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.[95] 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 (prawns, shrimp and mole crabs). 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.[86] When adults take food back to the nest, they recognise their young by call, rather than visual identification.[38]

The common tern may attempt to steal fish from Arctic terns,[96] but might itself be harassed by kleptoparasitic skuas,[97] laughing gulls,[98] roseate terns,[99] or by other common terns while bringing fish back to its nest.[96] In one study, two males whose mates had died spent much time stealing food from neighbouring broods.[100]

Terns normally drink in flight, usually taking seawater in preference to freshwater, if both are available.[5] Chicks do not drink before fledging, reabsorbing water, and, like adults, excreting excess salt in a concentrated solution from a specialised nasal gland.[101][102] 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.[5]

Predators and parasites

Rats will take tern eggs, and may even store large numbers in caches,[103] and the American mink is an important predator of hatched chicks, both in North America, and in Scotland where it has been introduced.[75] The red fox can also be a local problem.[104] 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,[105][106] and gulls may take chicks.[107][108] Great horned owls and short-eared owls will kill both adults and chicks, and black-crowned night herons will also eat small chicks.[5][109] 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.[68]

The common tern hosts feather lice, which are quite different from those found in Arctic terns, despite the close relationship of the two birds.[110] 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.[111] A study of 75 breeding common terns found that none carried blood parasites.[112] Colonies have been affected by avian cholera and ornithosis,[5] and it is possible that the common tern may be threatened in the future by outbreaks of avian influenza to which it is susceptible.[89] 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.[113]

Status

Dark-billed Asian subspecies S. h. longipennis in Mooloolaba, Australia

The common tern is classed as least concern on the IUCN Red List.[1] 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 (11,300,000 sq mi). Breeding numbers have been estimated at 250,000–500,000 pairs, the majority breeding in Asia. Up to 140,000 pairs breed in Europe.[114] Fewer than 80,000 pairs breed in North America, with most breeding on the northeast Atlantic coast[115] and a declining population of less than 10,000 pairs breeding in the Great Lakes region.[116]

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.[5][104] 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.[89]

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.[89] 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.[89] Contamination with polychlorinated biphenyls (PCBs) resulted in enhanced levels of feminisation in male embryos, which seemed to disappear prior to fledging, with no effect on colony productivity,[117] but dichlorodiphenyldichloroethylene (DDE), which results from the breakdown of DDT, led to very low levels of successful breeding in some US locations.[5]

The common tern is one of the species to which the Agreement on the Conservation of African-Eurasian Migratory Waterbirds (AEWA) and the US-Canada Migratory Bird Treaty Act of 1918 apply.[118][119] 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.[120] The North American legislation is similar, although there is a greater emphasis on protection.[121]

See also

References

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Cited texts

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  • Bent, Arthur Cleveland (1921). Life Histories of North American Gulls and Terns: Order Longipennes. Washington, DC: Government Printing Office.
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  • Blomdahl, Anders; Breife, Bertil; Holmstrom, Niklas (2007). Flight Identification of European Seabirds. London: Christopher Helm. ISBN 0-7136-8616-2.
  • Brazil, Mark (2008). Birds of East Asia. London: Christopher Helm. ISBN 0-7136-7040-1.
  • Cocker, Mark; Mabey, Richard (2005). Birds Britannica. London: Chatto & Windus. ISBN 0-7011-6907-9.
  • Cuthbert, Francesca J; Wires, Linda R; Timmerman, Kristina (2003). Status Assessment and Conservation Recommendations for the Common Tern (Sterna hirundo) in the Great Lakes Region (PDF). U S Department of the Interior, Fish and Wildlife Service, Fort Snelling, Minnesota. Archived from the original (PDF) on 2014-11-27.
  • van Duivendijk, Nils (2011). Advanced Bird ID Handbook: The Western Palearctic. London: New Holland. ISBN 1-78009-022-6.
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  • Hilty, Steven L (2002). Birds of Venezuela. London: Christopher Helm. ISBN 0-7136-6418-5.
  • Hume, Rob (1993). The Common Tern. London: Hamlyn. ISBN 0-540-01266-1.
  • Hume, Rob; Pearson, Bruce (1993). Seabirds. London: Hamlyn. ISBN 0-600-57951-4.
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  • Watling, Dick (2003). A Guide to the Birds of Fiji and Western Polynesia. Suva, Fiji: Environmental Consultants. ISBN 982-9030-04-0.
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External links

Arctic tern

The Arctic tern (Sterna paradisaea) is a tern in the family Laridae. This bird has a circumpolar breeding distribution covering the Arctic and sub-Arctic regions of Europe, Asia, and North America (as far south as Brittany and Massachusetts). The species is strongly migratory, seeing two summers each year as it migrates along a convoluted route from its northern breeding grounds to the Antarctic coast for the southern summer and back again about six months later. Recent studies have shown average annual roundtrip lengths of about 70,900 km (44,100 mi) for birds nesting in Iceland and Greenland and about 90,000 km (56,000 mi) for birds nesting in the Netherlands. These are by far the longest migrations known in the animal kingdom. The Arctic tern flies as well as glides through the air. It nests once every one to three years (depending on its mating cycle); once it has finished nesting it takes to the sky for another long southern migration.

Arctic terns are medium-sized birds. They have a length of 28–39 cm (11–15 in) and a wingspan of 65–75 cm (26–30 in). They are mainly grey and white plumaged, with a red/orangish beak and feet, white forehead, a black nape and crown (streaked white), and white cheeks. The grey mantle is 305 mm, and the scapulae are fringed brown, some tipped white. The upper wing is grey with a white leading edge, and the collar is completely white, as is the rump. The deeply forked tail is whitish, with grey outer webs.

Arctic terns are long-lived birds, with many reaching fifteen to thirty years of age. They eat mainly fish and small marine invertebrates. The species is abundant, with an estimated one million individuals. While the trend in the number of individuals in the species as a whole is not known, exploitation in the past has reduced this bird's numbers in the southern reaches of its range.

The Arctic tern was known as sea swallow describing their slender shape as they swoop over the water.

Breezy Point, Queens

Breezy Point is a neighborhood in the New York City borough of Queens, located on the western end of the Rockaway peninsula, between Rockaway Inlet and Jamaica Bay to the north and the Atlantic Ocean to the west and south. The community is run by the Breezy Point Cooperative, in which all residents pay the maintenance, security, and community-oriented costs involved with keeping the community private. The cooperative owns the entire 500-acre (2 km2) community; residents own their homes and hold shares in the cooperative. Breezy Point is less urbanized than most of the rest of New York City.

Breezy Point Tip, to the west of the community, is part of Gateway National Recreation Area, which is run by the National Park Service. This isolated, 200-acre (0.81 km2) area includes an ocean-facing beach, a shoreline on Jamaica Bay, sand dunes, and marshland. It is a breeding spot for the piping plover, least tern, black skimmer, American oystercatcher and common tern.Breezy Point is part of Queens Community District 14 and is patrolled by the 100th Precinct of the New York City Police Department. Its ZIP Code is 11697.

Cape Freels

Cape Freels is a headland on the island of Newfoundland, in the province of Newfoundland and Labrador, Canada, and the location of a community of the same name. This cape, located at the northern extremity of Bonavista Bay, is not to be confused with another Cape Freels which is located at the southern extremity of the Avalon Peninsula of Newfoundland.

The cape was named Ilha de Freyluis as early as 1506. The Portuguese translation is a derivation of the island of Brother Lewis. The area around the cape is the location of a Beothuk camping site. Radiocarbon dating of the artifacts place them between AD 200-700.For navigational safety a gas lamp was erected at Gull Island, Cape Freels in 1924.

'Cape Freels Coastline and Cabot Island' is designated as an Important Bird Area by BirdLife International. The area provides important nesting habitat for seabirds such as Common Eider, Common Murre, Razorbill, Common Tern, and Arctic Tern.

Forster's tern

The Forster's tern (Sterna forsteri) is a tern in the family Laridae. The genus name Sterna is derived from Old English "stearn", "tern", and forsteri commemorates the naturalist Johann Reinhold Forster.It breeds inland in North America and winters south to the Caribbean and northern Central America.

This species is rare but annual in western Europe, and has wintered in Ireland and Great Britain on a number of occasions. No European tern winters so far north.

This species breeds in colonies in marshes. It nests in a ground scrape and lays two or more eggs. Like all white terns, it is fiercely defensive of its nest and young.

The Forster's tern feeds by plunge-diving for fish, but will also hawk for insects in its breeding marshes. It usually feeds from saline environments in winter, like most Sterna terns. It usually dives directly, and not from the "stepped-hover" favoured by the Arctic tern. The offering of fish by the male to the female is part of the courtship display.

This is a small tern, 33–36 cm (13–14 in) long with a 64–70 cm (25–28 in) wingspan. It is most similar to the common tern. It has pale grey upperparts and white underparts. Its legs are red and its bill is red, tipped with black. In winter, the forehead becomes white and a characteristic black eye mask remains. Juvenile Forster's terns are similar to the winter adult. The call is a harsh noise like a black-headed gull.

This species is unlikely to be confused with the common tern in winter because of the black eye mask, but is much more similar in breeding plumage. Forster's has a grey centre to its white tail, and the upperwings are pure white, without the darker primary wedge of the common tern.

Hilfield Park Reservoir

Hilfield Park Reservoir is a 76.3 ha (189 acres) Local Nature Reserve (LNR) between Bushey and Elstree in Hertfordshire. It is owned by Affinity Water and managed by the Herts and Middlesex Wildlife Trust (HMWT). It is the oldest LNR in Hertfordshire, declared in 1969.This large reservoir has a wide range of bird species. It is of national importance for pochards, tufted ducks and common tern, and it is also significant for mallards, shovellers and teal. The margins have marshy areas with many breeding birds and marsh plants, such as reedmace and reed canarygrass. In 2011 an eastern crowned warbler was caught and ringed, only the second to be found in Britain.

The site is kept locked and access is restricted to HMWT members.

John Michael Cullen

Professor John Michael Cullen (14 December 1927 – 23 March 2001) was an Australian ornithologist, of English origin. Mike Cullen began his academic career by studying mathematics at Wadham College, Oxford, but later switched to zoology, spending time at the Edward Grey Institute of Field Ornithology while investigating the ecology of marsh tits. He subsequently achieved his PhD with Niko Tinbergen with a study of the behaviour of the common tern on the Farne Islands off the coast of Northumberland.

In 1976 he moved to Australia, to Monash University in Melbourne, Victoria. There he was involved in an investigation of Abbott's booby on Christmas Island which was threatened by phosphate mining. He served on the Field Investigation Committee of the Royal Australasian Ornithologists Union (RAOU) for which he organised the Rolling Bird Survey project. However, he is best known for long-term studies of the little penguin at Phillip Island and in Port Phillip Bay at St Kilda, in collaboration with Pauline Reilly and others.

Lilla Värtan

Lilla Värtan (Swedish: Smaller Värtan) or simply Värtan is a strait in Stockholm, Sweden. Separating mainland Stockholm from the island and municipality Lidingö, it stretches from Blockhusudden in the south to Stora Värtan in the north, and is joined by the Stocksundet mid-way. Two bridges, collectively called Lidingöbron ("Lidingö Bridge") stretch over the strait.

While most of the coasts surrounding the strait are occupied by industries and the ferry terminals and oil tanks in the harbour area of Värtahamnen, natural beaches are found in both the southern and the northern end of the strait and the strait forms part of the Royal National City Park.Most common fish species are Baltic herring, sea trout, and salmon. Stationary predator fishes, e.g. northern pike and perch, are exposed to raised levels of mercury.The area is considered an important wintering location for several birds species, including swans, Eurasian coot, common pochard, tufted duck, black-headed gull, lesser black-backed gull, gadwall, and common tern. The strait constitutes an important locale, especially ice-free winters, besides the lake Isbladskärret on Djurgården.Vegetation on the shore lines of Lilla Värtan includes alder, purple loosestrife, common valerian, yellow loosestrife, reed canary-grass, tall fescue, lesser periwinkle, and giant knotweed.Levels of heavy metals and organic waste are high in the bottom silt and nutrient levels high in the water. The level of phosphorus was 29 µg/l in 2005.

Norderoog

Norderoog (Halligen Frisian: Noorderuug, Danish: Nørreog) is one of the ten German halligen islands of the North Frisian Islands in the Wadden Sea, which is part of the North Sea off the coast of Germany. A part of Hooge municipality, the island belongs to the Nordfriesland district.It is only temporarily inhabited by a bird warden from March to October. The refuge hut at the northeastern end is called Jens Wand Hütte, which is built on stilts to protect it from flooding. A former terp had been washed away. It has been the site of several ecological studies.

Piiukaarelaid

Piiukaarelaid (alternatively: Piiukaare laid and Piiulaid) is a small, uninhabited islet in the Baltic Sea belonging to the country of Estonia. Piiukaarelaid has an approximate area of 8.5 hectares and a circumference of 1.8 kilometers and is administered by the village of Mereäärse, Varbla Parish, Pärnu County. The islet is fully protected as part of the Varbla Islets Landscape reserve (Estonian: Varbla laidude maastikukaitseala), and is an important breeding site for 54 species of birds, including: the velvet scoter, the little tern, the red-backed shrike, the curlew, the common tern, the Arctic tern, the redshank, the northern shoveler, the gadwall, the black-tailed godwit, the Greylag goose the tufted duck, the mute swan, the common gull, the goosander, the common eider, the lapwing, and others.

Raudna Nature Reserve

Raudna Nature Reserve is a nature reserve situated in southern Estonia, in Viljandi County.

Raudna Nature Reserve protects a former quarry, now a lake, which functions as an important resting-place for migratory birds, as well as feeding and breeding ground for them. On the little islands in the lake, birds like coot and red-necked grebe make their nests. Other species of protected birds often found in the area includes little ringed plover, common tern and Eurasian curlew. The flora includes different species of orchids.

Riether Werder

The Riether Werder, also Riethscher Werder (Polish Ostrów), is an island in the Neuwarper See, a bay in the Stettin Lagoon. It is the only island in the lagoon on German territory.

The first recorded mention of the island dates to the year 1252, when Duke Barnim I of Pomerania gifted this island along with other possessions to Eldena Abbey. It was then given it the Slavic name Wozstro. The present name of the island is derived from the village of Rieth on the southern shore of the bay.

The island belongs to the district of Vorpommern-Greifswald in the German state of Mecklenburg-Vorpommern and lies in the extreme northeast of Germany. It has national importance as a bird island. It is 0.79 km² in area and lies about a kilometre from the south and west shore of the Neuwarper See. The sea border with Poland runs immediately past the eastern tip of the island.

Rare bird species such as the common tern and the snipe may be encountered here. White-tailed eagle, Montagu's harrier, marsh harrier, red kite, black kite, kestrel, hobby, honey buzzard and common buzzard are also found here. Access to the island is forbidden; like the west shore of the Neuwarper See it is part of the Altwarp Inland Dunes, Neuwarper See and Riether Werder Nature Reserve.

River Ythan

The Ythan is a river in the north-east of Scotland rising at Wells of Ythan near the village of Ythanwells and flowing south-eastwards through the towns of Fyvie, Methlick and Ellon before flowing into the North Sea near Newburgh, in Formartine.

The lower reach of the river is known as the Ythan Estuary, a Special Protection Area for conservation, particularly the breeding ground of three tern species (common tern, little tern and Sandwich tern) (Lumina, 2004).

The River Ythan has a length of 60 km and a catchment area of 680 km2. As figures of the discharge, 6 m3/s or 7,2 m3/s are given.

South American tern

The South American tern (Sterna hirundinacea) is a species of tern found in coastal regions of southern South America, including the Falkland Islands, ranging north to Peru (Pacific coast) and Brazil (Atlantic coast). It is generally the most common tern in its range. The smaller, highly migratory common tern closely resembles it. The specific epithet refers to the "swallow-like" forked tail feathering.

Sterna

Sterna is a genus of terns in the bird family Laridae. Sterna is derived from Old English "stearn" which appears in the poem The Seafarer; a similar word was used to refer to terns by the Frisians. It used to encompass most "white" terns indiscriminately, but mtDNA sequence comparisons have recently determined that this arrangement is paraphyletic. It is now restricted to the typical medium-sized white terns occurring near-globally in coastal regions.

Target Rock National Wildlife Refuge

The Target Rock National Wildlife Refuge is located just east of the village of Lloyd Harbor, New York, on the north shore of Long Island, 25 miles (40 km) east of New York City. It is managed by the U.S. Fish and Wildlife Service as part of the Long Island National Wildlife Refuge Complex.

The 80-acre (32 ha) refuge is composed of mature oak-hickory forest, a one-half-mile (0.80 km) rocky beach, a brackish pond, and several vernal ponds. The land and waters support a variety of songbirds (particularly warblers during spring migration), mammals, shorebirds, fish, reptiles and amphibians. During the colder months, diving ducks are common offshore, while harbor seals occasionally use the beach and nearby rocks as resting sites. New York State and federally protected piping plover, least tern, and common tern depend on the refuge's rocky shore for foraging and rearing young.

The spring bloom at Target Rock is a reminder of its days as a garden estate, with flowering rhododendrons and mountain laurel.

Tern

Terns are seabirds in the family Laridae that have a worldwide distribution and are normally found near the sea, rivers, or wetlands. Terns are treated as a subgroup of the family Laridae which includes gulls and skimmers and consist of eleven 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.

Vaindloo

Vaindloo (alternately: Vaindloo saar, Swedish: Stenskär) is a small island located in the Gulf of Finland in the Baltic Sea. It belongs to Estonia, and is the northernmost land point of the country.

Vaindloo has an area of 6.2 hectares (15 acres) and marks the northernmost point of the nation of Estonia, situated 26 kilometers (16 mi) north of the Estonian mainland. The island is administered by the community of Vihula in Lääne-Viru County and is an important breeding sanctuary for such birds as the common tern, Arctic tern, Tengmalm's owl, great tit, purple sandpiper, shore lark, great grey shrike, yellowhammer and others.Vaindloo is also notable for its functioning lighthouse, called the Vaindloo tuletorn, it was built in 1871 and is managed by the Estonian Maritime Administration. A previous lighthouse constructed of timber was erected on Vaindloo in 1718. In addition to the lighthouse, there is a station of the Estonian Border Guard with a 50-meter-high (160 ft) observation tower and a radar on the island.

Whiskered tern

The whiskered tern (Chlidonias hybrida) is a tern in the family Laridae. The genus name is from Ancient Greek khelidonios, "swallow-like", from khelidon, "swallow". The specific hybridus is Latin for hybrid; Pallas thought it might be a hybrid of white-winged black tern and common tern, writing "Sterna fissipes [Chlidonias leucopterus] et Hirundine [Sterna hirundo] natam”.This bird has a number of geographical races, differing mainly in size and minor plumage details.

C. h. hybrida breeds in warmer parts of Europe and Asia. The smaller-billed and darker C. h. delalandii is found in east and south Africa, and the paler C. h. javanicus from Java to Australia.

The tropical forms are resident, but European and Asian birds winter south to Africa and the Indian Subcontinent.

This species breeds in colonies on inland marshes, sometimes amongst black-headed gulls, which provide some protection. The scientific name arises from the fact that this, the largest marsh tern, show similarities in appearance to both the white Sterna terns and to black tern.

White-fronted tern

The white-fronted tern (Sterna striata) is a species of tern from New Zealand and Australia. It is the most common tern of New Zealand. It rarely swims, apart from bathing, despite having webbed feet. The species is protected.

White-fronted terns feed in large flocks by plunge diving on shoals of smelt and pilchards which have been driven to the surface by larger fish and are easily caught. Like all terns they fly with their heads and bills pointing down to see their prey.

Breeding is between October and January in large colonies on rocky cliffs and offshore islands. The species was long known to breed on islands in Bass Strait, north of Tasmania, but the colonies there were not discovered until 1979. Many of the birds winter in south-eastern Australia, especially juveniles.

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