{{Short description|Genus of birds}} {{automatic taxobox | name = Sheathbills | image = Snowy Sheathbill.jpg | image_caption = Snowy sheathbill (''C.{{nbsp}}albus'') | display_parents = 2 | parent_authority = Bonaparte, 1832 | taxon = Chionis | authority = J.R. Forster, 1788<ref>''Enchiridion Historiae Naturali'': [http://biodiversitylibrary.org/page/25760780 37].</ref> | type_species = ''Vaginalis alba'' | type_species_authority = Gmelin, JF, 1789 | subdivision_ranks = Species | subdivision = ''Chionis albus'' <br /> ''Chionis minor'' }}
The '''sheathbills''' ('''''Chionis''''') is a genus of birds in the monotypic family '''Chionidae''' of the wader order Charadriiformes; the family consists of one genus with two species. They breed on subantarctic islands and the Antarctic Peninsula, and the snowy sheathbill migrates to the Falkland Islands and coastal southern South America in the southern winter; they are the only bird family endemic as breeders to the Antarctic region.<ref name=Firefly>{{cite book | last = Mead | first = Christopher J. | author2 = Richford, Andrew S. | year = 2003 | chapter = Sheathbills | editor = Perrins, Christopher | editor-link = Chris Perrins | title = The Firefly Encyclopedia of Birds | publisher = Firefly Books | pages = [https://archive.org/details/fireflyencyclope0000unse/page/256 256–257] | isbn = 1-55297-777-3 | url-access = registration | url = https://archive.org/details/fireflyencyclope0000unse/page/256 }}</ref> They are also the only Antarctic birds without webbed feet.
==Taxonomy== The genus ''Chionis'' was introduced in 1788 by the German naturalist Johann Reinhold Forster.<ref>{{ cite book | last=Forster | first=Johann Reinhold | author-link=Johann Reinhold Forster | date=1788 | title=Enchiridion historiae naturali inserviens, quo termini et delineationes ad avium, piscium, insectorum et plantarum adumbrationes intelligendas et concinnandas, secundum methodum systematis Linnaeani continentur | language=Latin | location=Halae | publisher=Prostat apud Hemmerde et Schwetschke | page=37 | url=https://www.biodiversitylibrary.org/page/25760780 }}</ref> The type species is the snowy sheathbill, ''Chionis albus''.<ref>{{ cite book | editor-last=Peters | editor-first=James Lee | editor-link=James L. Peters | year=1934 | title=Check-List of Birds of the World | volume=2 | publisher=Harvard University Press | place=Cambridge, Massachusetts | page=308 | url=https://www.biodiversitylibrary.org/page/14483121 }}</ref> The genus name is from the Ancient Greek {{Transliteration|grc|khiōn}} meaning snow.<ref>{{cite book | last=Jobling | first=James A. | year=2010| title=The Helm Dictionary of Scientific Bird Names | publisher=Christopher Helm | location=London | isbn=978-1-4081-2501-4 | page=101 | url=https://archive.org/stream/Helm_Dictionary_of_Scientific_Bird_Names_by_James_A._Jobling#page/n101/mode/1up }}</ref>
Genetic studies of the order Charadriiformes show the sheathbills to be a sister group of the thick-knees of the family Burhinidae. These two groups together are in turn a sister group to the Recurvirostridae-Haematopodidae and Charadriidae.<ref>Christidis and Boles, p. 128</ref> Recent research on the Magellanic plover (''Pluvianellus socialis'') of southern South America has indicated it too may be classified within the sheathbill family.<ref>Christidis and Boles, p. 132</ref><ref>{{cite journal | last1 = Paton | first1 = Tara A. | last2 = Baker | first2 = Allan J. | year = 2006 | title = Sequences from 14 mitochondrial genes provide a well-supported phylogeny of the Charadriiform birds congruent with the nuclear RAG-1 tree | journal = Mol. Phylogenet. Evol. | volume = 39 | issue = 3| pages = 657–667 | doi = 10.1016/j.ympev.2006.01.011 | pmid = 16531074 }}</ref><ref>{{cite journal | last1 = Paton | first1 = T.A. | last2 = Baker | first2 = A.J. | last3 = Groth | first3 = J.G. | last4 = Barrowclough | first4 = G.F. | year = 2003 | title = RAG-1 sequences resolve phylogenetic relationships within charadriiform birds | journal = Mol. Phylogenet. Evol. | volume = 29 | pages = 268–278 | doi = 10.1016/S1055-7903(03)00098-8 | pmid = 13678682 }}</ref><!--If this is accepted, much of this page will have to be moved to a new ''Chionis'' page.--><!--Or a Chionidae page be created instead-->
==Description== Sheathbills have white plumage including a thick layer of down, with only the face and leg colours distinguishing the two species. They appear plump and dove-like, but are believed to be similar to the ancestors of the modern gulls and terns.{{Citation needed|date=March 2025}} There is a rudimentary spur on the "wrist" or carpal joint, as in plovers. The skin around the eye is bare, as is the skin above the bill, which has carbuncular swellings. They derive their English common name from the horny sheath (rhamphotheca) which partially covers the upper mandible of their stout bills.<ref name=Firefly/> They are commonly known in the Antarctic as "Mutts" because of their call which is a soft "Mutt, mutt, mutt".{{Citation needed|date=March 2025}}
The two species are: {{Species table |genus= Chionis |authority-name=J.R. Forster|authority-year= 1788 |species-count=two|no-note=y|narrow-percent=75}}
{{Species table/row |name=snowy sheathbill|binomial=Chionis albus |image=File:Chionis blanc - Pale-faced Sheathbill.jpg|image-size=180px |image-alt= |authority-name=Gmelin, JF|authority-year= 1789|authority-not-original=yes |range= Antarctica, the Scotia Arc, the South Orkneys, and South Georgia |range-image=File:Chionis albus map.svg |range-image-size=180px |size= |habitat= |hunting= |iucn-status= LC |population= |direction= |subspecies= }}
{{Species table/row |name=black-faced sheathbill|binomial=Chionis minor |image=File:Chionis minor à Crozet.jpg|image-size=180px |image-alt= |authority-name=Hartlaub|authority-year= 1841|authority-not-original= |range= South African territory of the Prince Edward Islands, the French territories of the Crozet Islands and Kerguelen Islands, and the Australian territory of Heard Island. |range-image= |range-image-size=180px |size= |habitat= |hunting= |iucn-status= LC |population= |direction= |subspecies= }} {{Species table/end}}
==Behaviour== Sheathbills habitually walk on the ground, somewhat like rails. They fly only when alarmed or in migration, looking like pigeons.<ref name=Firefly/>
===Food and feeding=== The sheathbills are scavengers and opportunistic feeders, consuming invertebrates, faeces, and carrion—including the afterbirth of seals and their stillborn pups—between the tidelines. They also take chicks and eggs from penguins and cormorants.<ref name="HBW1996">{{cite book |last=Burger |first=A. E. |editor-last=del Hoyo |editor-first=J. |editor2-last=Elliott |editor2-first=A. |editor3-last=Sargatal |editor3-first=J. |title=Handbook of the Birds of the World |volume=3: Hoatzin to Auks |chapter=Family Chionidae (Sheathbills) |publisher=Lynx Edicions |location=Barcelona |year=1996 |pages=546–555 |isbn=84-87334-20-2}}</ref>
The bird has also been observed to directly pilfer milk from the teats of elephant seals.<ref>{{cite journal |last1=Sazima |title=Cleaner birds: an overview for the Neotropics |journal=Museu de Zoologia, Universidade Estadual de Campinas |volume=10 |issue=4 |url=https://www.scielo.br/j/bn/a/cx4vzMJnKQMKjDftsVXb5sp/?format=pdf&lang=en}}</ref><ref>{{cite book |last1=Chester |title=Antarctic birds and seals |date=1993 |url=https://www.abebooks.com/Antarctic-Birds-Seals-Pocket-Guide-Chester/769351226/bd}}</ref><ref>{{cite journal |title=Antarctica's Milk-Stealing, Grudge-Holding Seabirds |journal=Boston NPR |date=2016 |url=http://archives.wbur.org/thewildlife/2016/03/29/antarcticas-milk-stealing-grudge-holding-seabirds/}}</ref>
===Breeding=== During the penguin breeding season, which is also the sheathbill breeding season, pairs of sheathbills in penguin colonies maintain territories covering a number of penguin nests. Two mated sheathbills often work together to harass adult penguins, nimbly avoiding their attempts to peck; they gain access to the eggs or chicks or steal the krill that the adult penguins regurgitate to feed their chicks. Near the few human settlements of the region, they boldly forage for offal. Because of this diet, they spend a good deal of time cleaning themselves.<ref name=Firefly/>
They lay two or three blotchy white eggs in crevices or rock cavities.<ref name=EoB>{{cite book |editor=Forshaw, Joseph|author= Harrison, Colin J.O.|year=1991|title=Encyclopaedia of Animals: Birds|publisher= Merehurst Press|location=London|pages= 109|isbn= 1-85391-186-0}}</ref> The nests are lined messily with seaweed, stones, feathers, guano, bones, and occasionally plastic trash; even dead chicks may not be removed. Incubation lasts 28 to 32 days, and the young fledge 50 to 60 days later.<ref name=Firefly/>
==References== {{Reflist}}
{{Charadriiformes|C.|state=collapsed}} {{taxonbar|from=Q734677}}
==External links== *[https://web.archive.org/web/20131004043356/http://ibc.lynxeds.com/family/sheathbills-chionidae Sheathbill videos] on the Internet Bird Collection
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