Editing Common tern (section)

==Description==
[[File:Sterna hirundo in Finland.jpg|thumb|Adult ''S. h. hirundo'' in the harbour of [[Jyväskylä]], Finland]]
[[File:Sterna hirundo -Nantucket National Wildlife Refuge, Massachusetts, USA -head-8.jpg|thumb|Adult ''S. h. hirundo'' in breeding plumage at [[Nantucket National Wildlife Refuge]], Massachusetts]]

The nominate subspecies of the common tern is {{convert|31|-|35|cm|abbr=on}} long, including a {{convert|6|-|9|cm|abbr=on}} fork in the tail, with a {{convert|77|-|98|cm|abbr=on}} wingspan. It weighs {{convert|110|-|141|g|abbr=on}}.<ref name=olsen/> 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.<ref name=hume21>Hume (1993) pp. 21–29.</ref> The common tern's upper wings are pale grey, but as the summer wears on, the dark feather shafts of the outer [[flight feather]]s 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.<ref name=vinicombe/> 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.<ref name=vinicombe/> The upper wings have an obvious dark area at the front edge of the wing, the carpal bar. Terns that have not bred successfully may [[Moulting|moult]] into non-breeding adult plumage beginning in June, though late July is more typical, with the moult suspended during migration. There is also some geographical variation; [[California]]n birds are often in non-breeding plumage during migration.<ref name=olsen/>

Juvenile common terns have pale grey upper wings with a dark carpal bar. The crown and [[nape]] are brown, and the forehead is ginger, wearing to white by autumn. The upper parts are ginger with brown and white scaling, and the tail lacks the adult's long outer feathers.<ref name =olsen/> 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.<ref name= harrison/>

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 {{convert|10|-|30|m|abbr=on}} above the water in a following wind. Unless migrating, normally it stays below {{convert|100|m|abbr=on}}, and averages {{convert|30|kph|abbr=on}} in the absence of a tail wind.<ref name= nisbet/> Its average flight speed during the [[nocturnal]] migration flight is {{convert|43|-|54|kph|abbr=on}}<ref name= alerstam>{{cite journal |last=Alerstam |first=T |year=1985 |title=Strategies of migratory flight, illustrated by Arctic and common terns, ''Sterna paradisaea'' and ''Sterna hirundo'' |journal=Contributions to Marine Science |volume=27 |issue=supplement on migration: mechanisms and adaptive significance |pages=580–603}}</ref> at a height of {{convert|1000|-|3000|m|abbr=on}}.<ref name= nisbet/>

===Moult===
[[File:Commonternprimary.jpg|thumb|Detail of primary feather|300px]]

Juveniles moult into adult plumage in its first October; first the head, tail, and body plumage is replaced, mostly by February, then the wing feathers. The [[Flight feather#Primaries|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 forty and ninety per cent of feathers being replaced.<ref name=olsen/> 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 because of this feather ageing process.<ref name=hume21/>

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.<ref name= bridgeeaton>{{cite journal |last=Bridge |first=Eli S |author2=Eaton, Muir D |year=2005 |title=Does ultraviolet reflectance accentuate a sexually selected signal in terns? |journal=Journal of Avian Biology |volume=36 |issue=1 |pages=18–21 |doi=10.1111/j.0908-8857.2005.03470.x}}</ref> Experienced females favour mates which best show their [[Fitness (biology)|fitness]] through the quality of their wing feathers.<ref name=bridge2004>{{cite journal |last=Bridge |first=Eli S |author2=Nisbet, Ian C T |year=2004 |title=Wing molt and assortative mating in Common Terns: a test of the molt-signaling hypothesis |journal=Condor |volume=106 |issue=2 |pages=336–343 |doi=10.1650/7381|s2cid=84393348 |doi-access=free }}</ref> Rarely, a very early moult at the nesting colony is linked to breeding failure, both the onset of moult and reproductive behaviour being linked to falling levels of the [[hormone]] [[prolactin]].<ref name=BB105>{{cite journal |last=Braasch |first=Alexander |author2=Garciá, Germán O |year=2012 |title=A case of aberrant post-breeding moult coinciding with nest desertion in a female Common Tern |journal=British Birds |volume=105 |pages=154–159}}</ref>

===Similar species===


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.<ref name= BB86/> Arctic terns have greyer underparts than the common variety, which makes 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 the Arctic stay white throughout the northern summer. All the [[flight feather]]s of the Arctic tern are translucent against a bright sky, only the four innermost wing feathers of the common tern share this property.<ref name=BB86/><ref name=vanDuivendijk/> 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.<ref name=vinicombe/> The bill of an adult common tern is orange-red with a black tip, except in black-billed ''S.{{nbs}}h.{{nbs}}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.<ref name=BB86>{{cite journal |last=Hume |first=Rob A |year=1993 |title=Common, Arctic and Roseate Terns: an identification review |journal=British Birds |volume=86 |pages=210–217}}</ref>

In the breeding areas, the roseate tern can be distinguished by its pale plumage, long, mainly black bill and very long tail feathers.<ref name=vanDuivendijk>van Duivendijk (2011) pp. 200–202.</ref> 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.<ref name=olsen2>Olsen & Larsson (1995) pp. 69–76.</ref> In flight, the roseate's heavier head and neck, long bill and faster, stiffer wingbeats are also characteristic.<ref name=Blomdahl>Blomdahl et al. (2007) p. 340.</ref> It feeds further out to sea than the common tern.<ref name=olsen2/> 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.<ref name=olsen3>Olsen & Larrson (1995) pp. 103–110.</ref>

In the wintering regions, there are also confusion species, including the Antarctic tern of the southern oceans, the [[South American tern]], the [[Australasia]]n [[white-fronted tern]] and the [[white-cheeked tern]] of the [[Indian Ocean]]. The plumage differences due to "opposite" breeding seasons may aid in identification. 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 [[Flight feather#Tertials|tertials]], obvious on the closed wing and in flight.<ref name=Enticott196>Enticott & Tipling (2002) p. 196.</ref><ref name=SASOL>Sinclair et al. (2002) p. 212.</ref> 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.<ref name=peru>Schulenberg et al. (2010) p. 154.</ref> 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.<ref name=Enticott192>Enticott & Tipling (2002) p. 192.</ref> 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.<ref name=Simpson/> The white-cheeked tern is smaller, has uniform grey upperparts, and in breeding plumage is darker above with whiter cheeks.<ref name=grimmett>Grimmett et al. (1999) pp. 140–141.</ref>

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".<ref name=vinicombe>Vinicombe et al. (1990) pp. 133–138.</ref> [[Hybrid (biology)|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.<ref name=olsen/>

===Voice===
Common terns have a wide repertoire of calls, which have a lower pitch than the equivalent calls of Arctic terns. The most distinctive sound is the [[Alarm signal|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.<ref name=hume68/> A down-slurred {{not a typo|''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.<ref name=hume68>Hume (1993) pp. 68–75.</ref>

Parents and chicks can locate one another by call, and [[sibling]]s also recognise each other's vocalisations from about the twelfth day from hatching, which helps to keep the brood together.<ref name= sibling>{{cite journal | last= Burger | first= Joanna | author2= Gochfeld, Michael | author3= Boarman, William I | year= 1988 | title= Experimental evidence for sibling recognition in Common Terns (''Sterna hirundo'') | journal= Auk | volume= 105 | issue= 1 | pages= 142–148 | jstor= 4087337 | url= http://sora.unm.edu/node/24524 | doi= 10.1093/auk/105.1.142 | access-date= 22 February 2013 | archive-date= 21 October 2020 | archive-url= https://web.archive.org/web/20201021191648/https://sora.unm.edu/node/24524 | url-status= live | url-access= subscription }}</ref><ref name = Stevenson/>