Fairy tern

{{Short description|Species of bird}} {{Distinguish|text = the white tern (''Gygis alba''), also known as the fairy tern}} {{Speciesbox | image = Sterna nereis - Little Swanport.jpg | status = VU | status_system = IUCN3.1 | status_ref = <ref name="iucn status 19 November 2021">{{cite iucn |author=BirdLife International |date=2018 |title=''Sternula nereis'' |volume=2018 |article-number=e.T22694691A132568135 |doi=10.2305/IUCN.UK.2018-2.RLTS.T22694691A132568135.en |access-date=19 November 2021}}</ref> | genus = Sternula | species = nereis | authority = Gould, 1843 | subdivision_ranks = Subspecies | subdivision = ''Sternula nereis davisae'' ''Sternula nereis exsul'' ''Sternula nereis nereis'' | synonyms = ''Sterna nereis'' }}

The '''fairy tern''' ('''''Sternula nereis''''') is a small tern which is native to the southwestern Pacific. It is listed as vulnerable by the IUCN and the New Zealand subspecies is critically endangered. Fairy terns live in colonies along the coastlines and estuaries of Australia, New Zealand, and New Caledonia, feeding largely on small, epipelagic schooling fishes, breeding in areas close to their feeding sites. They have a monogamous mating system, forming breeding pairs in which they mate, nest, and care for offspring.

There are three subspecies: * Australian fairy tern, ''Sternula nereis nereis'' (Gould, 1843) – breeds in Australia * New Caledonian fairy tern, ''Sternula nereis exsul'' (Mathews, 1912) – breeds in New Caledonia * New Zealand fairy tern / Tara iti, ''Sternula nereis davisae'' (Mathews & Iredale, 1913) – breeds in northern New Zealand The three subspecies are distinguished by geographical range, and slight morphological differences.<ref name=":4" /> Gene flow between subspecies is little to none.<ref>{{Cite journal |last1=Baling |first1=Marleen |last2=Brunton |first2=Dianne H. |date=2022 |title=Structured phylogeography and restricted gene flow among populations of Fairy Tern ( Sternula nereis ) across Australasia: implications for the endangered New Zealand population |url=https://onlinelibrary.wiley.com/doi/10.1111/ibi.13048 |journal=Ibis |language=en |volume=164 |issue=3 |pages=800–808 |doi=10.1111/ibi.13048 |hdl=10652/5674 |issn=0019-1019|hdl-access=free }}</ref>

==Description== The fairy tern is a small tern with a white body and light bluish-grey wings. A small black patch extends no further than the eye and not as far as the bill. In the breeding plumage both the beak and the legs are yellowish-orange. During the rest of the year the black crown is lost, being mostly replaced by white feathers, and the beak becomes black at the tip and the base. The sexes look alike and the plumage of immature birds is similar to the non-breeding plumage. The total length of the fairy tern is about {{convert|25|cm|0|abbr=on}}.<ref name=Tasmania>{{cite web |url=http://www.parks.tas.gov.au/?base=17091 |title=Fairy Tern: ''Sterna nereis''|publisher=Tasmania Parks and Wildlife Service |access-date=2013-12-17}}</ref>

==Status== Formerly classified as a species of least concern by the IUCN,<ref name="BLI" /> recent research shows that its numbers have been decreasing rapidly throughout its range; the New Zealand subspecies has been on the brink of extinction for decades. The fairy tern was consequently uplisted to vulnerable status in 2008.<ref name="BLI" /> The New Zealand fairy tern has numerous breeding areas, largely incorporating the upper-north region of the North Island. In 2011, there were only about 42 known individuals. With a breeding program in place by the New Zealand Department of Conservation, the population was estimated in 2020 at 40.<ref>{{cite news |last1=Smith |first1=Anneke |date=23 August 2018 |title=Rarest bird in the country gets a helping hand |url=https://www.rnz.co.nz/news/national/364728/rarest-bird-in-the-country-gets-a-helping-hand |work=RNZ |language=en-nz}}</ref> Since then, their breeding sites have been reduced to only four consistent locations, limited to the South of the Northland Peninsula.<ref name=":2" /> In 2023, less than 40 individuals and 9 breeding pairs of the New Zealand fairy tern remained, the subspecies becoming a high priority for conservation.<ref name=":2" />

==Behaviour==

=== Feeding === Fairy terns are surface plungers, feeding on fish that shoal just under the water surface. To forage, fairy terns hover between five and fifteen metres above the water to search for prey, then carrying out a descending aerial dive beak-first towards the water.<ref name=":0">{{cite journal |last1=Ismar |first1=Stefanie M. H. |last2=Trnski |first2=Tom |last3=Beauchamp |first3=Tony |last4=Bury |first4=Sarah J. |last5=Wilson |first5=David |last6=Kannemeyer |first6=Robyn |last7=Bellingham |first7=Mark |last8=Baird |first8=Karen |title=Foraging ecology and choice of feeding habitat in the New Zealand Fairy Tern Sternula nereis davisae |journal=Bird Conservation International |date=March 2014 |volume=24 |issue=1 |pages=72–87 |id={{ProQuest|1611063475}} |doi=10.1017/S0959270913000312 }}</ref> They then spread their wings and tails just above the water surface, submerging only their bills and heads to catch their prey.<ref name=":0" /> This foraging technique means that they catch prey no deeper than around eight centimetres under the water surface, allowing them to make use of shallow waters, such as tidal pools.<ref name=":0" /> Fairy terns seldom go far out to sea but are often to be seen where predatory fish are feeding on shoals of small fish.

Fairy tern diets consist predominantly of pelagic schooling fishes.<ref name=":6">{{cite journal |last1=Greenwell |first1=C.N. |last2=Tweedley |first2=J.R. |last3=Moore |first3=G.I. |last4=Lenanton |first4=R.C.J. |last5=Dunlop |first5=J.N. |last6=Loneragan |first6=N.R. |title=Feeding ecology of a threatened coastal seabird across an inner shelf seascape |journal=Estuarine, Coastal and Shelf Science |date=December 2021 |volume=263 |article-number=107627 |doi=10.1016/j.ecss.2021.107627 |bibcode=2021ECSS..26307627G }}</ref><ref name=":1">{{cite thesis |last=Greenwell |first=Claire |date=2021 |title=Life history, ecology, and population dynamics of the Australian Fairy Tern and implications for their conservation |url=https://researchportal.murdoch.edu.au/esploro/outputs/991005543999507891 }}{{page needed|date=October 2024}}</ref> For instance, Australian fairy terns mostly eat blue sprat, hardyheads, and garfishes.<ref name=":1" /> Similarly, New Zealand fairy terns have a diet made up of common estuarine fish, namely gobies and flounders, responsible for most of their consumed biomass, as well as shrimps, comprising up to 21% of their diet.<ref name=":0" /> Little research has been done on the diet of New Caledonian fairy terns, but given their foraging technique, it is likely that they too forage for small marine fish that school just below the water surface. Fairy terns also consume crustaceans, molluscs and some plant material.<ref name="Tasmania" /> Diets may vary according to location, time of day, developmental stage and breeding season.<ref name=":6" />

=== Breeding === Breeding takes place in the spring in colonies on sheltered beaches on the mainland or on offshore islands. The nest is just above high-water mark and is a scrape in the sand. Fairy terns have a monogamous mating system, forming pair bonds in which they provision food, mate, nest, and care for offspring.<ref name=":4" /> One or two eggs are laid and both parents share the incubation and care of the chicks and have occasionally been seen providing post-fledging parental care.<ref name=BLI>{{cite web |url=http://www.birdlife.org/datazone/speciesfactsheet.php?id=3281 |title=Species factsheet: ''Sterna nereis'' |publisher=BirdLife International |access-date=2013-12-17}}</ref> Breeding success is low.<ref name=":5" /><ref name=":1" />

==== Breeding Season ==== Fairy terns breed during spring, with courtship beginning in September, and nesting occurring largely from November to February.<ref name=":4" /> Their breeding season largely overlaps with the spawning season for much of their prey,<ref name=":6" /> allowing fairy terns to make use of a higher abundance of food that is required for courtship provisioning, energy for breeding, and feeding offspring. Females breed at around three years old, while males breed from age two.<ref name=":4" /> Each year, fairy terns develop breeding plumage, where their bills, legs, and feet become brighter and darker, and the dark colouration on their heads extends from forehead to nape.<ref name=":4" /> This plumage signals sexual maturity, and elicits the courtship process.

==== Courtship ==== After developing breeding plumage, fairy terns begin the courtship process. During pair-formation, fairy terns exhibit ritualised courtship behaviours. Courtship displays are typically exhibited by male fairy terns to attract females during mate selection. Fairy tern courtship behaviours include aerial displays as well as ground displays. Aerial displays may be social, involving cooperative exhibitions of dynamic flight patterns.<ref name=":1" /> Ground displays involve catching and exhibiting fish, to signal foraging ability. As observed in many avian species, courtship displays function to indicate mate quality in order to facilitate reproductive success.<ref>{{Cite book |last=Bastock |first=Margaret |url=https://www.taylorfrancis.com/books/9780203794562 |title=Courtship: An Ethological Study |date=2018-05-04 |publisher=Routledge |isbn=978-0-203-79456-2 |edition=1 |language=en |doi=10.4324/9780203794562}}</ref> In fairy terns, courtship displays are essential to breeding, which will not occur without them.<ref name=":1" /> Another key element of courtship in Fairy Terns is the exchange of fish, which is initially essential before copulation can occur in breeding pairs.<ref name=":1" /><ref name=":4" /> Males will provision food to the female, which persists throughout the breeding season. Male provisioning behaviour is thought to function to demonstrate the parental ability of the male in courtship.<ref name=":1" />

==== Pair Bonds ==== Following courtship, fairy terns form pair bonds.<ref name=":1" /><ref name=":4" /><ref name=":2" /> In these pairs, fairy terns prospect potential nesting sites within the colony territory. Once chosen, pairs will frequently visit their nesting habitat, feed together, and mate frequently. High levels of fidelity are generally observed. Although fairy terns are typically observed forming single pair bonds during the mating season, multiple mating pairs and copulation have been observed in Australian fairy terns.<ref name=":1" /> Here, males will typically guard their partner during breeding, in attempt to prevent polyandrous copulation outside of their breeding pair. Fairy terns stay in their breeding pairs throughout nesting, both investing in the biparental care of chicks and eggs together.

==== Nesting ==== [[File:DOC Fairy Tern photos 11.jpg|thumb|Tara iti (New Zealand fairy tern) chicks in nest.<ref>{{Cite web |title=New Zealand fairy tern/tara iti |url=https://www.doc.govt.nz/nature/native-animals/birds/birds-a-z/nz-fairy-tern-tara-iti/ |access-date=2024-10-14 |website=Department of Conservation |language=en-nz}}</ref>]] thumb|Fairy tern eggs camouflage with shells. Fairy terns nest in low lying sand, eggs and young camouflaging with surrounding shells, shingle or gravel. They construct their nests by scraping the sand with their legs, rotating in a circle until they have dug sand from all directions.<ref name=":4" /> Fairy terns may create several nests before their final selection of nest choice.<ref name=":1" /> Fairy terns have been observed to nest in different location types, including seaside bays, estuary mouths, sheltered lagoons and saltwater lakes.<ref name=":1" /><ref name=":4" /><ref name=":5" /> Given their gregarious nature, nest selection is influenced by social facilitation, where the observation of nesting success in conspecifics of their colony will direct fairy terns to also nest in that location.<ref>{{Cite journal |last1=Greenwell |first1=Claire N. |last2=Dunlop |first2=J.N. |last3=Admiraal |first3=R. |last4=Loneragan |first4=N.R. |date=2020 |title=The secret life of Fairy Terns: breeding chronology and life history observations of Sternula nereis nereis in south-western Australia. |url=https://www.publish.csiro.au/PC/PC20056 |journal=Pacific Conservation Biology |volume=27 |issue=2 |pages=143–154|doi=10.1071/PC20056 }}</ref> Colonies will often abandon nest location once the breeding season ends, driven by changing availability of food, predators, and vegetation.<ref name=":1" />

Nest location may be related to feeding site, where fairy terns will select areas that allow them to easily and quickly access food for their young while nesting.<ref>{{Cite journal |last1=Jeffries |first1=Darryl S. |last2=Bull |first2=Leigh S. |last3=Lagnaz |first3=Eliane G. |last4=Wilson |first4=David S. |last5=Wilson |first5=Peter |last6=Zimmerman |first6=Rangi L. |last7=Pulham |first7=Gwenda A. |date=2016 |title=New Zealand fairy tern (Sternula nereis davisae) foraging behaviour at Te Arai Stream |url=https://www.birdsnz.org.nz/wp-content/uploads/2021/12/Jeffries_et_al._2016.pdf |journal=The Ornithological Society of New Zealand Inc. |volume=63 |pages=42–45}}</ref> Like many other terns, fairy terns often nest in sandy beach areas with little vegetation, allowing them to detect predators easily, and nest close to feeding sites. However, too little vegetation leaves fairy terns with insufficient shelter, making them more vulnerable to weather and avian predation.<ref name=":4" /> So, choice of nest site is influenced by evolutionary trade-offs pressured by a need for safety and food. Another key aspect of nest site selection is an abundance of shell cover, which fairy terns will preferentially choose. This preference seemingly functions to increase camouflage and avoid predator detection, given their colouration which likely evolved to matched the white, orange and black shelled areas in which they nest.<ref name=":4" />

During nesting, female fairy terns rarely leave the nesting site.<ref name=":1" /> Males supply their partner with food throughout the nesting and incubation periods, though this behaviour decreases over time until the eggs hatch, when provisioning increases once more to care for the offspring. Male provisional feeding gives the female nutritional support, allowing her to invest more in nesting and attend the eggs. Males may experience a decrease in body mass during this period, given the energy expense of provisioning behaviour.<ref name=":4" /> This food provisioning behaviour, typically carried out by males, is therefore important in increasing breeding success.<ref name=":4" />

==== Breeding Success ==== Fairy tern clutch size varies from one to three eggs,<ref name=":1" /><ref name=":4" /><ref name=":7" /> with clutch size of one or two being most common. Larger broods typically occur with more experienced pairs, and only when resources are abundant.<ref name=":1" /> The second egg is typically laid one to four days after the first.<ref name=":4" /> The incubation period lasts approximately twenty-two days.<ref name=":1" /><ref name=":7" />

Studies on New Caledonian fairy terns find breeding success to be quite low.<ref name=":5" /><ref name=":7" /><ref name=":8" /> Chick mortality may occur due to several factors including predation by other avian species, tidal flooding, egg failure, adverse weather and parental desertion.<ref name=":7" /><ref name=":9">{{Cite journal |last1=Roberts |first1=James J. |last2=Brunton |first2=Dianne H. |last3=Clement |first3=Hannah |last4=Harmer |first4=Aaron M.T. |date=2024-04-02 |title=Observations of chick feeding rates and parental defensive responses to disturbance at nests in the critically endangered New Zealand fairy tern/tara iti ( Sternula nereis davisae ) |journal=New Zealand Journal of Zoology |language=en |volume=51 |issue=2 |pages=175–185 |doi=10.1080/03014223.2023.2256239 |issn=0301-4223|doi-access=free }}</ref> Breeding success is also hindered by nest disturbance from conspecifics nesting nearby.<ref name=":9" /> The "grieving parent" syndrome has been observed in New Zealand fairy terns, where parents who experience offspring failure will kill the chicks of a nearby nest.<ref name=":4" /> Surveys of New Caledonian fairy terns find breeding success to be highly impacted by adverse weather, where almost all nests across 2 years were destroyed by weathering.<ref name=":5" /> In 2020, breeding success in New Caledonian fairy terns was less than 15%.<ref name=":7" /> The low breeding success of New Caledonian fairy terns is similar to that of the endangered New Zealand fairy tern.<ref name=":8" /> Further research is required to reliably establish breeding success in the Australian and New Zealand subspecies, though it is thought to be very low given their high vulnerability to tidal flooding and predation.<ref name=":1" /><ref name=":9" /> However, adult survival is considerably higher, where fairy terns are able to mate for multiple breeding seasons, giving hope to the continuation of their species.<ref name=":1" />

==== Parental Behaviour ==== As observed in other tern species, both male and female fairy terns contribute equally towards parental care.<ref name=":4" /> In their pair bonds, both males and females feed their offspring. Males continue to provide food to the female as well as young. When there is only one chick, males feed the chick more than the female, and at night, the females care for the chicks.<ref name=":4" /> Chick feeding rates vary considerably between nests, and decrease with disturbance as parents engage more in defensive behaviour.<ref name=":9" /> Parents are highly attentive towards chicks particularly in the first few days after hatching.<ref name=":1" /><ref name=":10" /> Chicks are not left unattended until at least fourteen days of age; fledgling occurs at approximately day twenty-three.<ref name=":4" />

Parental behaviour is influenced by a variety of components. Increased wind speed is associated with increased time spent with young,<ref name=":4" /> presumably to increase protection to favour offspring survival. Feeding of young occurs the most frequently approximately three hours past low tide, while foraging occurs at high tide.<ref name=":4" />

Increased aggression, both conspecific and intraspecific, is observed when parents are with young.<ref name=":4" /><ref name=":9" /> In defence, to protect their offspring, fairy terns will display aggressive behaviours towards perceived potential predators (mammalian, avian, and human), as well as intruding conspecifics.<ref name=":4" /><ref name=":9" /> This aggressive behaviour will be exhibited upon intrusion within seventy-five metres of the nest site.<ref name=":4" /> Parents will also extend their wings over chicks to provide protection to young.<ref name=":10" />

== Threats ==

=== Predation === Fairy terns are predated on by small mammals, which may eat adults, chicks and eggs.<ref name=":2">{{Cite web |title=New Zealand fairy tern/tara iti |url=https://www.doc.govt.nz/nature/native-animals/birds/birds-a-z/nz-fairy-tern-tara-iti/ |access-date=2024-10-12 |website=Department of Conservation |language=en-nz}}</ref><ref name=":3">{{Cite journal |last1=Greenwell |first1=Claire N. |last2=Calver |first2=Michael C. |last3=Loneragan |first3=Neil R. |date=2019 |title=Cat Gets Its Tern: A Case Study of Predation on a Threatened Coastal Seabird |journal=Animals |language=en |volume=9 |issue=7 |page=445 |doi=10.3390/ani9070445 |doi-access=free |pmid=31315191 |pmc=6681120 |issn=2076-2615}}</ref><ref name=":4">{{Cite thesis |last=Treadgold |date=2000 |title=Behavioural Ecology of the endangered New Zealand Fairy Tern (Tara-iti) Sterna nereis davisae: implications for management |url=https://mro.massey.ac.nz/items/498aeaf9-0de0-49b3-b856-cc97be875362 |journal=Massey University}}</ref> In Australia, the presence of semi-wild cats threatens the already declining population of Australian fairy terns.<ref name=":3" /> Likewise, in New Zealand, non-native invasive mammalian species including rats, mustelids, hedgehogs and cats, predate on fairy terns.<ref name=":2" /><ref name=":4" /> As with fairy terns, non-native mammalian predation is a common issue for vulnerable endemic birds in New Zealand, and is a key focus for conversation.

Fairy tern chicks and eggs are also at risk of avian predation.<ref name=":7">{{Cite journal |last1=Villard |first1=Pascal |last2=Hunt |first2=Gavin |last3=Coll |first3=Jonathan |last4=Cassan |first4=Jean-Jérome |date=2020-10-01 |title=Breeding biology of Fairy Terns on Magone Islet, New Caledonia |url=https://www.tandfonline.com/doi/full/10.1080/01584197.2020.1830705 |journal=Emu - Austral Ornithology |language=en |volume=120 |issue=4 |pages=360–364 |doi=10.1080/01584197.2020.1830705 |bibcode=2020EmuAO.120..360V |issn=0158-4197|url-access=subscription }}</ref><ref name=":5">{{Cite journal |last1=Barré |first1=Nicolas |last2=Baling |first2=Marleen |last3=Baillon |first3=Nathalie |last4=Le Bouteiller |first4=Aubert |last5=Bachy |first5=Pierre |last6=Chartendrault |first6=Vivien |last7=Spaggiari |first7=Jérôme |date=2012 |title=Survey of Fairy Tern Sterna Nereis Exsul in New Caledonia |url=http://www.marineornithology.org/PDF/40_1/40_1_31-38.pdf |journal=Marine Ornithology |volume=40 |pages=31–38 |doi=10.5038/2074-1235.40.1.958 }}</ref> Specifically, birds including harrier hawks and black backed gulls will eat chicks and eggs.<ref name=":2" /><ref name=":4" /> The only defence against predation for fairy tern chicks is their cryptic colouration, which allows them to camouflage with seashells that surround their nests.<ref name=":4" /><ref name=":10">{{Cite journal |last1=Greenwell |first1=C.N. |last2=Sullivan |first2=D. |last3=Goddard |first3=N. |last4=Bedford |first4=F. |last5=Douglas |first5=T.K. |date=2021 |title=Application of a novel banding technique and photographic recapture to describe plumage development and behaviour of juvenile fairy terns. |url=https://search.informit.org/doi/10.3316/informit.752734791049554 |journal=Australian Field Ornithology |volume=38 |pages=49–55 |doi=10.20938/afo38049055 |via=Informit|url-access=subscription }}</ref> This contributes to a high level of chick mortality that threatens the decreasing population of fairy terns, particularly for the endangered New Zealand and New Caledonian subspecies.<ref name=":8">{{Cite journal |last1=Baling |first1=Marleen |last2=Jeffries |first2=Darryl |last3=Barré |first3=Nicolas |last4=Brunton |first4=Dianne H. |date=2009 |title=A survey of Fairy Tern ( Sterna nereis ) breeding colonies in the Southern Lagoon, New Caledonia |url=https://www.tandfonline.com/doi/full/10.1071/MU08047 |journal=Emu - Austral Ornithology |language=en |volume=109 |issue=1 |pages=57–61 |doi=10.1071/MU08047 |bibcode=2009EmuAO.109...57B |issn=0158-4197 |via=Tandfonline|url-access=subscription }}</ref>

=== Human Disturbance === Human disturbance poses a great threat to fairy terns. Particularly during the breeding season, human activity puts fairy terns at risk of further population decrease, disrupting nesting and breeding behaviours to ultimately reduce breeding success.<ref name=":4" /><ref name=":5" /> The New Zealand Department of Conservation warns of the danger of human activation including dog walking, drone use, bonfires, vehicular beach use, horse riding, and recreational beach activities in fairy tern breeding areas. These disturbances have been known to not only disrupt breeding behaviours, but to scare fairy terns away from their nests, causing fairy terns to abandon their eggs, leaving them vulnerable to predation as well as embryo death due to thermal exposure.<ref name=":2" /> For this reason, conversation efforts are being made to reduce human disturbance towards fairy terns.

==References== {{Reflist}}

==External links== {{Commons category|Sternula nereis}} {{Wikispecies|Sternula nereis}} * [http://www.birdlife.org/datazone/species/index.html?action=SpcHTMDetails.asp&sid=3281&m=0 BirdLife Species Factsheet]

fairy tern fairy tern Category:Birds of New Caledonia Category:Birds of South Australia Category:Birds of Tasmania Category:Birds of Victoria (state) Category:Birds of Western Australia fairy tern fairy tern