Editing Common tern (section)

==Status==
[[File:Common Tern96.ogg|thumb|Dark-billed Asian subspecies ''S.{{nbs}}h.{{nbs}}longipennis'' in [[Mooloolaba, Queensland|Mooloolaba]], Australia]]

The common tern is classed as [[least concern]] on the [[IUCN Red List]].<ref name="iucn status 12 November 2021" /> It has a large population of 1.6 to 3.3{{nbs}}million mature individuals and a huge breeding range estimated at {{convert|84300000|km2|abbr=on}}. Breeding numbers have been estimated at a quarter to half a million pairs, the majority breeding in Asia. About 140 thousand pairs breed in Europe.<ref name=Enticott>Enticott (2002) p. 194.</ref> Fewer than eighty thousand pairs breed in North America, with most breeding on the northeast Atlantic coast<ref name=kress>{{cite journal |author=Kress, Stephen W |author2=Weinstein, Evelyn H |author3=Nisbet, Ian C T |author4=Shugart, Gary W |author5=Scharf, William C |author6=Blokpoel, Hans |author7=Smith, Gerald A |author8=Karwowski, Kenneth |author9=Maxwell, George R |author10=Chapdelaine, Gilles |author11=Montevecchi, William A |author12=Lock, Anthony R |author13=Smith, Carol F |author14=Miller, Eileen |author15=Spendelow, Jeffrey A |author16=Gochfeld, Michael |author17=Burger, Joanna |author18=Erwin, R Michael |year=1983 |title=The status of tern populations in northeastern United States and adjacent Canada |journal=Colonial Waterbirds |volume=6 |pages=84–106 |doi=10.2307/1520976|jstor=1520976}}</ref> and a declining population of less than ten thousand pairs breeding in the [[Great Lakes region]].<ref name=cuthbert1>Cuthbert (2003) p. 1.</ref>

In the nineteenth century, the use of tern feathers and wings in the [[Hatmaking|milliner]]y 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 movement|conservation]] organizations.<ref name=nisbet/><ref name=jncc>{{cite web |title=Common Tern ''Sterna hirundo'' |work=Latest population trends |url=http://jncc.defra.gov.uk/page-2895 |publisher=Joint Nature Conservation Committee, (JNCC) |access-date=25 January 2012 |archive-date=7 March 2012 |archive-url=https://web.archive.org/web/20120307011220/http://jncc.defra.gov.uk/page-2895 |url-status=live }}</ref> Although some Eurasian populations are stable, numbers in North America have fallen by more than seventy per cent in the last forty years, and there is an overall negative trend in the global estimates for this species.<ref name = BirdLife/>

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.<ref name = BirdLife/> Breeding success may be enhanced by the use of floating nest rafts, manmade 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.<ref name = BirdLife/> Contamination with [[polychlorinated biphenyl]]s (PCBs) resulted in enhanced levels of [[Feminization (biology)|feminisation]] in male embryos, which seemed to disappear prior to fledging, with no effect on colony productivity,<ref name=hart>{{cite journal |last=Hart |first=Constance A |author2=Nisbet, Ian C T |author3=Kennedy, Sean W |author4=Hahn, Mark E |year=2003 |title=Gonadal feminization and halogenated environmental contaminants in Common Terns (''Sterna hirundo''): evidence that ovotestes in male embryos do not persist to the prefledgling stage |journal=Ecotoxicology |volume=12 |issue=1–4 |pages=125–140 |url=http://www.whoi.edu/science/B/people/mhahn/Hart_Ecotox_proofs.pdf |doi=10.1023/A:1022505424074 |pmid=12739862 |bibcode=2003Ecotx..12..125H |s2cid=21308753 |access-date=19 February 2012 |archive-date=21 July 2012 |archive-url=https://web.archive.org/web/20120721023432/http://www.whoi.edu/science/B/people/mhahn/Hart_Ecotox_proofs.pdf |url-status=live }}</ref> but [[dichlorodiphenyldichloroethylene]] (DDE), which results from the breakdown of [[DDT]], led to very low levels of successful breeding in some US locations.<ref name=nisbet/>

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.<ref name=AEWA>{{cite web|title=Annex 2: Waterbird species to which the Agreement applies |work=Agreement on the conservation of African-Eurasian migratory Waterbirds (AEWA) |url=http://www.unep-aewa.org/documents/agreement_text/eng/pdf/aewa_agreement_text_annex2.pdf |publisher=UNEP/ AEWA Secretariat |access-date=25 January 2012 |archive-url=https://web.archive.org/web/20110728144028/http://www.unep-aewa.org/documents/agreement_text/eng/pdf/aewa_agreement_text_annex2.pdf |archive-date=28 July 2011}}</ref><ref name=1918treaty>{{cite web |title=List of Migratory Birds |work=Birds protected by the Migratory Bird Treaty Act |url=http://www.fws.gov/migratorybirds/RegulationsPolicies/mbta/mbtandx.html |publisher=US Fish and Wildlife Service |access-date=25 January 2012 |archive-date=7 June 2019 |archive-url=https://web.archive.org/web/20190607221124/https://www.fws.gov/migratorybirds/RegulationsPolicies/mbta/mbtandx.html |url-status=live }}</ref> Parties to the AEWA 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.<ref name=AEWAintro>{{cite web|title=Introduction |work=African-Eurasian Waterbird Agreement |url=http://www.unep-aewa.org/about/introduction.htm |publisher=UNEP/ AEWA Secretariat |access-date=25 January 2012 |archive-url=https://web.archive.org/web/20120211082252/http://www.unep-aewa.org/about/introduction.htm |archive-date=11 February 2012}}</ref> The North American legislation is similar, although there is a greater emphasis on protection.<ref name=law>{{cite web |title=Migratory Bird Treaty Act of 1918 |work=Digest of Federal Resource Laws of Interest to the U. S. Fish and Wildlife Service |url=http://www.fws.gov/laws/lawsdigest/migtrea.html |publisher=U. S. Fish and Wildlife Service |access-date=25 January 2012 |archive-date=14 January 2012 |archive-url=https://web.archive.org/web/20120114090828/http://www.fws.gov/laws/lawsdigest/migtrea.html |url-status=live }}</ref>