Editing Common tern (section)

===Breeding===
[[File:Common tern (Sterna hirundo) fledgling Danube delta.jpg|thumb|Fledgling, Danube delta, Romania]]
Pairs are established or confirmed through aerial [[courtship display]]s in which a male and a female fly in wide circles up to {{convert|200|m|abbr=on}} 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.<ref name=hume91>Hume (1993) pp. 91–99.</ref> 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.<ref name=hume91/> The eggs may be laid on bare sand, gravel or soil, but a lining of debris or vegetation is often added if available,<ref name=snow779/> or the nest may be rimmed with seaweed, stones or shells. The saucer-shaped scrape is typically {{convert|4|cm|abbr=on}} deep and {{convert|10|cm|abbr=on}} across, but may extend to as much as {{convert|24|cm|abbr=on}} wide including the surrounding decorative material.<ref name=hume100>Hume (1993) pp. 100–111.</ref> Breeding success in areas prone to flooding has been enhanced by the provision of artificial mats made from [[Zostera|eelgrass]], which encourage the terns to nest in higher, less vulnerable areas, since many prefer the mats to bare sand.<ref name=Palestis>{{cite journal |last=Palestis |first=Brian G |year=2009 |title=Use of artificial eelgrass mats by saltmarsh-nesting Common Terns (''Sterna hirundo'') |journal=In Vivo |volume=30 |issue=3 |pages=11–16 |url=http://aquaticcommons.org/4729/1/eelgrass_mats.pdf |access-date=8 February 2012 |archive-date=27 May 2014 |archive-url=https://web.archive.org/web/20140527215847/http://aquaticcommons.org/4729/1/eelgrass_mats.pdf |url-status=live }}</ref> The common tern tends to use more nest material than roseate or Arctic terns, although roseate often nests in areas with more growing vegetation.<ref name= Lloyd>Lloyd et al. (2010) p. 207.</ref><ref name= Bent>Bent (1921) p. 252.</ref>

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 {{convert|5|m|abbr=on}} 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.<ref name= Fisher/>

The peak time for egg production is early May, with some birds, particularly first-time breeders, laying later in the month or in June.<ref name=ontario/><ref name=hume100/> The [[Clutch (eggs)|clutch]] size is normally three eggs; larger clutches probably result from two females laying in the same nest. Egg size averages {{convert|41|x|31|mm|abbr=on}}, although each successive egg in a clutch is slightly smaller than the first laid.<ref name=hume100/> The average egg weight is {{convert|20.2|g|abbr=on}}, of which five per cent is shell.<ref name=bto>{{cite web |title=Common Tern ''Sterna hirundo'' (Linnaeus, 1758) |work=BirdFacts |date=16 July 2010 |url=http://blx1.bto.org/birdfacts/results/bob6150.htm |publisher=British Trust for Ornithology (BTO) |access-date=9 February 2012 |archive-date=18 January 2012 |archive-url=https://web.archive.org/web/20120118042434/http://blx1.bto.org/birdfacts/results/bob6150.htm |url-status=live }}</ref> 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 (colour)|buff]], or pale brown, marked with streaks, spots or blotches of black, brown or grey which help to camouflage them.<ref name=hume100/> Incubation is by both sexes, although more often by the female, and lasts 21–22 days,<ref name=bto/> extending to 25{{nbs}}days if there are frequent disturbances at the colony which cause the adults to leave the eggs unattended;<ref name=hume100/> nocturnal predation may lead to incubation taking up to 34{{nbs}}days.<ref name=ontario/> 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.<ref name=nisbet/> Except when the colony suffers disaster, ninety per cent of the eggs hatch.<ref name=hume112/> The [[precocial]] [[down feather|downy]] chick is yellowish with black or brown markings,<ref name=hume100/> and like the eggs, is similar to the equivalent stage of the Arctic tern.<ref name=pearson121>Hume & Pearson (1993) pp. 121–124.</ref> The chicks [[fledge]] in 22–28 days,<ref name=bto/> usually 25–26.<ref name=snow779/> 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.<ref name=nisbet/><ref name=hume120>Hume (1993) pp. 120–123.</ref>

Like many terns, this species is very defensive of its nest and young, and will harass humans, dogs, [[muskrat]]s and most [[Diurnality|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.<ref name= Burger>{{cite journal |last=Burger |first=Joanna |author2=Shealer, D A |author3=Gochfeld, Michael |year=1993 |title=Defensive aggression in terns: discrimination and response to individual researchers |journal=Aggressive Behavior |volume=19 |issue=4 |pages=303–311 |doi=10.1002/1098-2337(1993)19:4<303::AID-AB2480190406>3.0.CO;2-P}}</ref> Nocturnal predators do not elicit similar attacks;<ref name= hunter>{{cite journal |last=Hunter |first=Rodger A |author2=Morris, Ralph D |year=1976 |title=Nocturnal predation by a Black-Crowned Night Heron at a Common Tern colony |journal=Auk |url=http://sora.unm.edu/node/22862 |volume=93 |issue=3 |pages=629–633 |jstor=4084965 |access-date=22 February 2013 |archive-date=28 May 2014 |archive-url=https://web.archive.org/web/20140528012956/https://sora.unm.edu/node/22862 |url-status=live }}</ref> colonies can be wiped out by rats, and adults desert the colony for up to eight hours when [[great horned owl]]s are present.<ref name= nisbet3>{{cite journal |last=Nisbet |first=Ian C T |author2=Welton, M |year=1984 |title=Seasonal variations in breeding success of Common Terns: consequences of predation |journal=Condor |volume=86 |issue=1 |pages=53–60 |url=http://sora.unm.edu/node/103417 |jstor=1367345 |doi=10.2307/1367345 |access-date=22 February 2013 |archive-date=28 May 2014 |archive-url=https://web.archive.org/web/20140528020412/https://sora.unm.edu/node/103417 |url-status=live |url-access=subscription }}</ref>

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.<ref name= hays>{{cite journal |last=Hays |first=H |year=1984 |title=Common Terns raise young from successive broods |journal=Auk |volume=101 |issue=2 |pages=274–280 |url=http://sora.unm.edu/sites/default/files/journals/auk/v101n02/p0274-p0280.pdf |doi=10.1093/auk/101.2.274 |access-date=22 February 2013 |archive-date=18 July 2015 |archive-url=https://web.archive.org/web/20150718134846/http://sora.unm.edu/sites/default/files/journals/auk/v101n02/p0274-p0280.pdf |url-status=live }}</ref> 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.<ref name=nisbet/><ref name="pearson121"/> The maximum documented lifespan in the wild is 23{{nbs}}years in North America<ref name=nisbet2>{{cite journal |last=Nisbet |first=Ian C T |author2=Cam, Emmanuelle |year=2002 |title=Test for age-specificity in survival of the Common Tern |journal=Journal of Applied Statistics |volume=29 |issue=1–4 |pages=65–83 |doi=10.1080/02664760120108467|bibcode=2002JApSt..29...65N |s2cid=62816201 }}</ref><ref name=austin2>{{cite journal |last=Austin |first=Oliver L Sr |year=1953 |title=A Common Tern at least 23 years old |journal=Bird-Banding |volume=24 |issue=1 |page=20 |url=http://sora.unm.edu/sites/default/files/journals/jfo/v024n01/p0020-p0020.pdf |access-date=22 February 2013 |archive-date=1 March 2014 |archive-url=https://web.archive.org/web/20140301131214/http://sora.unm.edu/sites/default/files/journals/jfo/v024n01/p0020-p0020.pdf |url-status=live }}</ref> and 33{{nbs}}years in Europe,<ref name=agebi>{{cite web |title=Longevity records for Britain & Ireland in 2010 |work=Online ringing report |url=http://blx1.bto.org/ring/countyrec/results2010/longevity.htm |publisher=British Trust for Ornithology (BTO) |archive-url=https://web.archive.org/web/20120228140407/http://blx1.bto.org/ring/countyrec/results2010/longevity.htm |archive-date=28 February 2012|access-date=11 February 2012}}</ref><ref name=ageeu>{{cite web |title=European Longevity Records |work=Longevity |url=http://www.euring.org/data_and_codes/longevity-voous.htm |publisher=Euring |access-date=11 February 2012 |archive-date=11 May 2015 |archive-url=https://web.archive.org/web/20150511193839/http://www.euring.org/data_and_codes/longevity-voous.htm |url-status=live }}</ref> but twelve years is a more typical lifespan.<ref name=bto/>

<gallery widths="200px" heights="165px">
File:Common Terns nesting.jpg|Nest site, [[Elliston, Newfoundland and Labrador]]
File:Batalla de golondrinas de mar (Sterna hirundo).jpg|Nest in the [[Ebro Delta]], Tarragona, Catalonia, Spain
File:Sterna hirundo MWNH 0472.JPG|Egg, Collection [[Museum Wiesbaden]]
File:Sterna hirundo -nest with three eggs-8.jpg|Three eggs in a nest on [[Great Gull Island]]
File:CommonTern-Chick.jpg|A chick on an island off the coast of Maine
File:Sterna hirundo -hovering to protect nest-8.jpg|Hovering and screaming to deter intruders on [[Great Gull Island]]
File:Sterna hirundo -Massachusetts, USA -juvenile-8.jpg|This autumn juvenile in Massachusetts has a white forehead, having lost the ginger colouration characteristic of younger birds.
</gallery>