View source for Hoatzin

{{Short description|Species of bird in South America}}
{{Speciesbox
|fossil_range = {{fossil range|Miocene|present}}
| name = Hoatzin
| image = Hoatzin (Opisthocomus hoazin) Rio Napo.jpg
| image_caption = in Ecuador
| image2 =Opisthocomus hoazin - Hoatzin XC251282.mp3
| status = LC
| status_system = IUCN3.1
| status_ref = <ref name="iucn status 4 June 2025">{{cite iucn |author=BirdLife International |date=2024 |title=''Opisthocomus hoazin'' |volume=2024 |article-number=e.T22684428A264086808 |doi=10.2305/IUCN.UK.2024-2.RLTS.T22684428A264086808.en |access-date=4 June 2025}}</ref>
| genus = Opisthocomus
| parent_authority = [[Johann Karl Wilhelm Illiger|Illiger]], 1811
| species = hoazin
| authority = ([[Philipp Ludwig Statius Müller|Müller]], 1776)
| range_map = Hoatzin (Opisthocomus hoazin) world.png
| range_map_caption = Range
| synonyms = 
''[[Phasianus]] hoazin'' <small>Müller, 1776</small>
}}

The '''hoatzin''' ({{IPAc-en|h|oʊ|ˈ|æ|t|s|ɪ|n}} {{respell|hoh|AT|sin}}){{NoteTag|{{IPAc-en|USalso|ˈ|w|ɑː|t|s|ɪ|n|,_|-|s|i:|n}} {{respell|WAHT|sin|,_|-seen}}<ref>{{Cite web |title=Definition of hoatzin |url=https://www.dictionary.com/browse/hoatzin |website=Dictionary.com |access-date=14 February 2022}}</ref><ref>{{Cite web|title=HOATZIN English Definition and Meaning |url=https://www.lexico.com/en/definition/hoatzin|website=Lexico Dictionaries |publisher=Lexico |access-date=2022-02-14|archive-url=https://web.archive.org/web/20220214091507/https://www.lexico.com/en/definition/hoatzin|archive-date=February 14, 2022  |language=en}}</ref>}} or '''hoactzin''' ({{IPAc-en|h|oʊ|ˈ|æ|k|t|s|ɪ|n}} {{respell|hoh|AKT|sin}}) ('''''Opisthocomus hoazin''''')<ref>{{Cite book|title=The Chambers Dictionary|publisher=Chambers|year=2003|isbn=0-550-10105-5|edition=9th|chapter=hoatzin}}</ref> is a species of tropical [[bird]] found in swamps, [[riparian]] forests, and [[mangrove]]s of the [[Amazon basin|Amazon]] and the [[Orinoco Basin|Orinoco]] basins in [[South America]]. It is the only [[extant taxon|extant]] species in the genus '''''Opisthocomus'''''<ref name = "HBW"/> which is the only extant genus in the [[Opisthocomidae]] family under the order of [[Opisthocomiformes]].<ref name=IOU>{{cite web| editor1-last=Gill | editor1-first=Frank | editor1-link=Frank Gill (ornithologist) | editor2-last=Donsker | editor2-first=David | editor3-last=Rasmussen | editor3-first=Pamela | editor3-link=Pamela Rasmussen | date=January 2023 | title= Hoatzin, New World vultures, Secretarybird, raptors | work=IOC World Bird List Version 13.1 | url=https://www.worldbirdnames.org/new/bow/raptors/ | publisher=International Ornithologists' Union | access-date=12 March 2023 }}</ref> Despite being the subject of intense debate by specialists, the [[taxonomy (biology)|taxonomic]] position of this family is still far from clear.

The hoatzin is notable for its chicks having primitive claws on two of their wing [[digit (anatomy)|digits]]; the species also is unique among birds, in possessing a [[digestive system]] that non-trivially supports the [[fermentation]] and the effective breaking-down of plant matter, a trait more commonly known from [[Herbivore|herbivorous]] [[ungulate]]-[[ruminant]] [[Mammalia|mammals]] and some [[primates]]. This bird is also the national bird of [[Guyana]], where the local name for this bird is '''Canje pheasant'''.

== Description ==

The hoatzin is [[pheasant]]-sized, with a total length of {{Convert|65|cm|0|abbr=on}}, and a long neck and small head. It has an unfeathered blue face with [[Maroon (color)|maroon]] eyes, and its head is topped by a spiky, [[rufous]] crest. The long, sooty-brown tail is bronze-green tipped with a broad whitish or [[buff (colour)|buff]] band at the end.<ref name="ADW">{{cite web |url=https://animaldiversity.org/accounts/Opisthocomus_hoazin/ |title=''Opisthocomus hoazin'' |last1=Williams |first1=Kellie |date=2001 |website=Animal Diversity Web |publisher=University of Michigan Museum of Zoology |access-date=12 March 2023 }}</ref> The upper parts are dark, sooty brown-edged buff on the [[wing covert]]s, and streaked buff on the mantle and nape. The underparts are buff, while the crissum (the undertail [[Covert feather|coverts]] surrounding the [[cloaca]]), [[Flight feather|primaries]], underwing coverts, and flanks are rich rufous-[[Chestnut (colour)|chestnut]], but this is mainly visible when the hoatzin opens its wings.

It is a noisy bird, and makes a variety of hoarse [[bird vocalization|calls]], including groans, croaks, hisses, and grunts.<ref name = "HBW">{{cite book | first = B. T. | last = Thomas | editor-last = Josep | editor-first = del Hoyo | editor2-last = Andrew | editor2-first = Jordi | editor3-last = Sargatal | editor3-first = Christie | contribution = Family Opisthocomidae (Hoatzins) | title = Handbook of the Birds of the World. Volume 3, Hoatzins to Auks | year = 1996 | pages = [https://archive.org/details/handbookofbirdso0001unse/page/24 24–32] | place = Barcelona | publisher = Lynx Edicions | isbn = 978-84-87334-20-7 | contribution-url = http://www.lynxeds.com/family-text/hbw-3-family-text-opisthocomidae-hoatzin | title-link = Handbook of the Birds of the World | access-date = 2010-11-03 | archive-date = 2014-03-07 | archive-url = https://web.archive.org/web/20140307214026/http://www.lynxeds.com/family-text/hbw-3-family-text-opisthocomidae-hoatzin }}</ref> These calls are often associated with body movements, such as wing spreading.

=== Young wing claws ===
Hoatzin chicks have two claws on each wing. Immediately after hatching, they can use these claws, and their oversized feet, to scramble around the tree branches without falling into the water.<ref name="DK" /> When predators such as the [[great black hawk]] attack a hoatzin nesting colony, the adults fly noisily about, trying to divert the predator's attention, while the chicks move away from the nest and hide among the thickets. If discovered, however, they drop into the water and swim under the surface to escape, then later use their clawed wings to climb back to the safety of the nest. This has inevitably led to comparisons to the fossil bird ''[[Archaeopteryx]]'', but the characteristic is rather an [[autapomorph]]y, possibly caused by an [[atavism]] toward the [[dinosaur]]ian finger claws, whose [[developmental genetics]] ("blueprint") presumably is still in the avian [[genome]] (furthermore, wing claws are not unique to hoatzins with even chickens still bearing them).<ref>{{cite journal |last1=Taylor |first1=Mike |title=Things to Make and Do, part 32: chicken wing claw |journal=Sauropod Vertebra Picture of the Week |date=3 October 2024 |doi=10.59350/9bsp1-r5551|doi-access=free }}</ref> Since ''Archaeopteryx'' had three functional claws on each wing, some earlier systematists speculated that the hoatzin was descended from it, because nestling hoatzins have two functional claws on each wing. Modern researchers, however, hypothesize that the young hoatzin's claws are of more recent origin, and may be a secondary adaptation from its frequent need to leave the nest and climb about in dense vines and trees well before it can fly.<ref name = "HBW" /> A similar trait is seen in [[turaco]]s, whose nestlings use claws on their wings to climb in trees.<ref>[https://books.google.com/books?id=gccfdvSUTg4C&dq=Turacos+claws+wings&pg=RA1-PA186 Birds of the World]</ref>

== Taxonomy, systematics, and evolution ==

The [[genus|generic]] name ''Opisthocomus'' comes from [[Ancient Greek]] {{lang|el|ὄπισθοκομος}} ''ópisthokomos'' derived from {{lang|el|ὄπισθε}} ''ópisthe'' ({{lang|el|ὄπισθεν}} ''ópisthen'' before a consonant) "behind" and {{lang|el|κόμη}} ''kómē'' "hair" altogether meaning "long hair behind" referring to its large [[crest (feathers)|crest]].<ref name = "HBW"/><ref>{{Cite book |last=Jobling |first=James A. |year=2010 |title=Helm Dictionary of Scientific Bird Names |location=London, UK |publisher=[[Christopher Helm]] |isbn=978-1-4081-3326-2 |page=282 |language=en |oclc=659731768 }}</ref>

The hoatzin was originally described in 1776 by German zoologist [[Philipp Ludwig Statius Müller|Statius Müller]]. Much debate has occurred about the hoatzin's relationships with other birds. Because of its distinctness, it has been given its own [[Family (biology)|family]], the Opisthocomidae, and its own [[Order (biology)|order]], the Opisthocomiformes. At various times, it has been allied with such [[taxa]] as the [[tinamou]]s, the [[Galliformes]] (gamebirds), the [[rail (bird)|rail]]s, the [[bustard]]s, [[seriema]]s, [[sandgrouse]], [[dove]]s, [[turaco]]s and other [[Cuculiformes]], and [[mousebird]]s.<ref name = "HBW"/> A [[whole genome sequencing]] study published in 2014 places the hoatzin as the [[Sister group|sister taxon]] of a [[clade]] composed of [[Gruiformes]] (cranes) and [[Charadriiformes]] (plovers).<ref name=Jarvis2014>{{cite journal | last1 = Jarvis | first1 = E.D. |display-authors=etal | year = 2014 | title = Whole-genome analyses resolve early branches in the tree of life of modern birds | journal = Science | volume = 346 | issue = 6215| pages = 1320–1331 | doi=10.1126/science.1253451 | pmid=25504713 | pmc=4405904| bibcode = 2014Sci...346.1320J }}</ref> Another genomic study in 2024 instead places it as the sister group to the [[Phaethoquornithes]] (containing numerous aquatic bird orders). The combined group was found to be sister to the [[Mirandornithes]] ([[Flamingo|flamingos]] and [[Grebe|grebes]]).<ref name=":0">{{Cite journal |last1=Wu |first1=Shaoyuan |last2=Rheindt |first2=Frank E. |last3=Zhang |first3=Jin |last4=Wang |first4=Jiajia |last5=Zhang |first5=Lei |last6=Quan |first6=Cheng |last7=Li |first7=Zhiheng |last8=Wang |first8=Min |last9=Wu |first9=Feixiang |last10=Qu |first10=Yanhua |last11=Edwards |first11=Scott V. |last12=Zhou |first12=Zhonghe |last13=Liu |first13=Liang |date=2024-02-20 |title=Genomes, fossils, and the concurrent rise of modern birds and flowering plants in the Late Cretaceous |journal=Proceedings of the National Academy of Sciences |language=en |volume=121 |issue=8 |article-number=e2319696121 |doi=10.1073/pnas.2319696121 |issn=0027-8424|doi-access=free |pmid=38346181 |pmc=10895254 |bibcode=2024PNAS..12119696W }}</ref>

In 2015, genetic research<ref>{{cite journal|last1=Prum|first1=Richard O.|last2=Berv|first2=Jacob S.|last3=Dornburg|first3=Alex|last4=Field|first4=Daniel J.|last5=Townsend|first5=Jeffrey P.|last6=Lemmon|first6=Emily Moriarty|last7=Lemmon|first7=Alan R.|title=A comprehensive phylogeny of birds (Aves) using targeted next-generation DNA sequencing|journal=Nature|date=7 October 2015|doi=10.1038/nature15697|volume=526|issue=7574|pages=569–573|pmid=26444237|bibcode=2015Natur.526..569P |s2cid=205246158}}</ref> indicated that the hoatzin is the last surviving member of a bird line that branched off in its own direction 64 million years ago, shortly after the extinction event that killed the nonavian dinosaurs.<ref>{{cite web|url=https://arstechnica.com/science/2015/10/new-study-rearranges-family-tree-of-birds/|title=New study rearranges family tree of birds|date=8 October 2015|last=Timmer|first=John|website=Ars Technica|publisher=Condé Nast|access-date=25 March 2023}}</ref> Another genetic study from 2024 instead suggested a [[Late Cretaceous]] origin (around 70 million years ago), but found that this early divergence is shared with a majority of extant bird orders, making it no more primitive than them.<ref name=":0" />
<gallery mode = packed heights = 160px>
Hoatzins in Brazil.jpg|in Brazil
Hoatzin (Opisthocomus hoazin) Rio Napo 2.jpg|in Ecuador
Hoatzin (Opisthocomus hoazin) head Rio Napo.jpg|in Ecuador
Hoatzin in Peru.jpg|in Peru
</gallery>
===Fossil record===
[[File:Hoatzin chick.jpg|thumb|upright|The newly hatched bird has claws on its thumb and first finger, enabling it to dexterously climb tree branches until its wings are strong enough for sustained flight.<ref>{{cite journal|doi=10.1111/j.1096-3642.1891.tb00045.x|last=Parker|first= W. K.|year=1891|url=https://www.biodiversitylibrary.org/page/31083330 |title=On the Morphology of a Reptilian Bird, ''Opisthocomus hoazin''|journal=Transactions of the Zoological Society of London|volume=13|pages=43–89|issue=2}}</ref> These claws disappear by the time the bird reaches adulthood.]]
With respect to other material evidence, an undisputed [[fossil]] record of a close hoatzin relative is specimen [[UCMP]] 42823, a single [[cranium]] backside.<ref>{{cite journal|last=Miller|first= Alden H. |year=1953|title= A fossil Hoatzin from the Miocene of Colombia|journal=[[Auk (journal)|Auk]]|volume=70|issue=4|pages=484–495|url=http://sora.unm.edu/sites/default/files/journals/auk/v070n04/p0484-p0489.pdf|doi=10.2307/4081360|jstor= 4081360 }}</ref> It is of [[Miocene]] origin{{NoteTag|Originally believed to be of Late Miocene age—from some 5–10 million years ago (Mya)—the bone was found in association with fossils of the extinct monkey ''[[Cebupithecia sarmientoi]]'' that today, usually is considered to be of the Early or Middle Miocene, possibly 18 but from at least some 12 Mya.}} and was recovered in the upper [[Magdalena River Valley]], [[Colombia]], in the well-known fauna of [[La Venta (Colombia)|La Venta]].<ref name="HBW"/> This has been placed into a distinct, less derived genus, ''[[Hoazinoides]]'', but clearly would be placed into the same family as the extant species. It markedly differs in that the cranium of the living hoatzin is characteristic, being much domed, rounded, and shortened, and that these autapomorphies were less pronounced in the Miocene bird. Müller discussed these findings in the light of the supposed affiliation of the hoatzins and the Galliformes, which was the favored hypothesis at that time but had been controversial almost since its inception. He cautioned, however, "that ''Hoazinoides'' by no means establishes a phyletic junction point with other galliforms" for obvious reasons, as we know today. Anything other than the primary findings of Müller are not to be expected in any case, as by the time of ''Hoazinoides'', essentially all modern bird families are either known or believed to have been present and distinct. Going further back in time, the Late [[Eocene]] or Early [[Oligocene]] (some 34 Mya) ''[[Filholornis]]'' from [[France]] has also been considered "proof" of a link between the hoatzin and the gamebirds.<ref name = "HBW"/> The fragmentary fossil ''[[Onychopteryx]]'' from the [[Eocene]] of [[Argentina]]{{Citation needed|date=February 2007}} and the quite complete, but no less enigmatic Early-Middle Eocene ([[Ypresian]]-[[Lutetian]], some 48 Mya) ''[[Foro (bird)|Foro panarium]]'' are sometimes used{{Citation needed|date=February 2007}} to argue for a hoatzin-cuculiform (including turacos) link. As demonstrated above, though, this must be considered highly speculative, if not as badly off the mark as the relationship with the [[Cracidae]] discussed by Miller.

The earliest record of the order Opisthocomiformes is ''[[Protoazin|Protoazin parisiensis]]'', from the latest [[Eocene]] (about 34 Mya) of [[Romainville]], [[France]]. The holotype and only known specimen is [[Naturhistorisches Museum Basel|NMB]] PG.70, consisting of partial [[coracoid]], partial [[scapula]], and partial [[wiktionary:pedal|pedal]] [[Phalanx bone|phalanx]]. According to the phylogenetic analysis performed by the authors, ''[[Namibiavis]]'', although later, is more basal than ''Protoazin''. Opisthocomiforms seem to have been much more widespread in the past, with the present South American distribution being only a relic. By the Early to Middle Miocene, they were probably extinct in Europe already, as formations dated to this time and representing fluvial or lacustrine palaeoenvironments, in which the hoatzin thrives today, have yielded dozens of bird specimens, but no opisthocomiforms. A possible explanation to account for the extinction of ''Protoazin'' between the Late Eocene and the Early Miocene in Europe, and of ''Namibiavis'' after the Middle Miocene of sub-Saharan Africa is the arrival of arboreal [[carnivora]]ns—predation which could have had a devastating effect on the local opisthocomiforms, if they were similarly poor flyers and had a similarly vulnerable nesting strategies as today's hoatzins. [[Felidae|Felids]] and [[Viverridae|viverrids]] first arrived in Europe from Asia after the [[Turgai Sea]] closed, marking the [[Grande Coupure|boundary]] between the Eocene and the [[Oligocene]]. None of these predators, and for the matter, no [[placental]] predator at all was present in South America before the [[Great American Interchange]] 3 Mya; this absence could explain the survival of the hoatzin there.<ref name=Protoazins>{{cite journal |year=2014 |title=Earliest and first Northern Hemispheric hoatzin fossils substantiate Old World origin of a "Neotropic endemic" |journal=Naturwissenschaften |volume=101 |issue=2 |pages= 143–8|doi=10.1007/s00114-014-1144-8|author=Gerald Mayr|author2=Vanesa L. De Pietri |pmid=24441712|bibcode=2014NW....101..143M |s2cid=14060583 }}</ref> In addition to being the earliest fossil record of an opisthocomiform, ''Protoazin'' was also the earliest find of one (1912), but it was forgotten for more than a century, being described only in 2014.

''[[Hoazinavis]]'' is an extinct genus of early opisthocomiforms from Late Oligocene and Early Miocene (about 24–22 Mya) deposits of Brazil. It was collected in 2008 from the Tremembé Formation of São Paulo, Brazil. It was first named by [[Gerald Mayr]], Herculano Alvarenga and Cécile Mourer-Chauviré in 2011 and the type species is ''[[Hoazinavis lacustris]]''.<ref name=Mayr2011>{{cite journal |first1=Gerald |last1=Mayr|first2=Herculano | last2 = Alvarenga |first3= Cécile |last3=Mourer-Chauviré |year=2011 |title=Out of Africa: Fossils shed light on the origin of the hoatzin, an iconic Neotropic bird |journal=Naturwissenschaften |volume=98 |issue=11 |pages=961–966 |doi=10.1007/s00114-011-0849-1 |pmid=21964974 |bibcode=2011NW.....98..961M |s2cid=24210185}}</ref>

''[[Namibiavis]]'' is another extinct genus of early opisthocomiforms from early Middle Miocene (around 16 Mya) deposits of Namibia. It was collected from Arrisdrift, southern Namibia. It was first named by Cécile Mourer-Chauviré in 2003 and the type species is ''[[Namibiavis senutae]]''.<ref name=Mayr2011/>

== Behavior ==
[[File:Rurrenabaque Bolivia - The Amazon.jpg|thumb|In flight, Bolivia]]

=== Feeding and habits ===
The hoatzin is a [[folivore]]—it eats the [[leaves]] (and to a lesser degree, the [[fruit]]s and [[flower]]s) of the plants that grow in its marshy and riverine habitat. It clambers around along the branches in its search for food. The hoatzin uses a leathery "bump" on the bottom of its crop to help balance its weight on the branches. The species was once thought to eat the leaves of only [[Araceae|arums]] and [[Avicennia|mangroves]], but the species is now known to consume the leaves of more than 50 botanical species. One study, undertaken in [[Venezuela]], found that the hoatzin's diet was 82% leaves, 10% flowers, and 8% fruit.<ref name = "HBW"/> Any feeding on insects or other animal matter is purely opportunistic or accidental.<ref>{{cite journal|doi=10.1126/science.245.4923.1236|last1=Grajal|first1=A.|first2=S. D.|last2= Strahl|first3=R. |last3=Parra|first4=M. G. |last4=Dominguez|first5= A. |last5=Neher|year=1989|title=Foregut fermentation in the Hoatzin, a Neotropical leaf-eating bird|journal=Science|volume= 245|issue=4923|pages=1236–1238|pmid=17747887|bibcode=1989Sci...245.1236G |s2cid=21455374}}.</ref>

One of this species' many peculiarities is its unique digestive system, which contains specialized bacteria in the front part of the gut that break-down and ferment the foliar material they consume (much like [[cattle]] and other [[ruminant]]s do). This process is more efficient than what has been measured in many other species of birds, with up to 70% of the plant fiber being digested.<ref name="DK">{{cite book | first = Tom | last = Jackson | year = 2015 | title = DK Eyewitness Books: The Amazon |url = https://books.google.com/books?id=hOjtBgAAQBAJ&pg=PA21 |location = New York, USA |publisher= [[DK Publishing]] |page= 21 |isbn= 978-1-4654-3566-8 }}</ref><ref name="DW">{{cite magazine |last=Parry |first=James |date=n.d. |title=Why does the hoatzin or 'stink bird' stink? |url=https://www.discoverwildlife.com/animal-facts/birds/why-does-the-hoatzin-or-stink-bird-stink/ |magazine=Discover Wildlife |publisher= Our Media Ltd |access-date=12 March 2023}}</ref><ref name="NYTimes.com 1995 u300">{{cite web |title=Strange Bird Must Think It's a Cow |website=[[The New York Times]] |date=1995-09-19 | url=https://www.nytimes.com/1995/09/19/science/strange-bird-must-think-it-s-a-cow.html | archive-url=https://web.archive.org/web/20150526144359/https://www.nytimes.com/1995/09/19/science/strange-bird-must-think-it-s-a-cow.html |archive-date=2015-05-26 |url-status=live |url-access=subscription |access-date=2023-11-25}}</ref> Unlike ruminants, however, which possess a [[rumen]] (a specialized, chambered stomach for bacterial fermentation), the hoatzin has an unusually large [[crop (anatomy)|crop]] that is folded into two chambers, with a large, multi-chambered lower esophagus. 

Serrations on the beak help cut leaves into smaller pieces before they are swallowed. Because they lack the teeth of mammals, hoatzins do not [[chewing the cud|chew the cud]]; instead, a combination of muscular pressure and abrasion by a "cornified" lining of the crop is used as an equivalent to [[remastication]], allowing fermentation and [[trituration]] to occur at the same site. The fermented foliage produces methane which the bird expels through burping. Its stomach chamber and gizzard are much smaller than in other birds. Its crop is so large as to displace the flight muscles and keel of the sternum, much to the detriment of its flight capacity. The crop is supported by a thickened skin callus on the tip of the sternum, which helps the bird support the crop on a branch during rest and while digesting its food. A hoatzin's meal takes up to 45 hours to pass through its body.<ref name="DW"/><ref>{{cite thesis|url=https://d-nb.info/976094037/34|title=Breeding Ecology and Related Life-History Traits of the Hoatzin, ''Opisthocomus hoazin'', in a Primary Rainforest Habitat|last=Müllner|first=Antje|institution=Julius Maximilian University of Würzburg|location=Würzburg, Germany|year=2004|access-date=25 November 2023|degree=PhD|via=Deutsche Nationalbibliothek}}</ref><ref>{{cite web|url=https://www.earth.com/news/stinking-bird-hoatzins/|title=Dirty birdies: The case of the stinking bird|website=earth.com|last=Pallardy|first=Richard|date=9 November 2018|access-date=25 November 2023}}</ref><ref>{{cite journal|journal=The Auk|volume=112|issue=1|pages=20–28|last=Grajal|first=Alejandro|year=1995|url=https://sora.unm.edu/sites/default/files/journals/auk/v112n01/p0020-p0028.pdf|title=Structure and Function of the Digestive Tract of the Hoatzin (''Opisthocomus hoazin''): A Folivorous Bird with Foregut Fermentation|doi=10.2307/4088763 |jstor=4088763 |via=Searchable Ornithological Research Archive, The University of New Mexico|access-date=25 November 2023}}</ref> With a body weight as low as {{convert|700|g|lbs}}, the adult hoatzin is the smallest known animal with foregut fermentation (the lower limit for mammals is about {{convert|3|kg|lbs|disp=or}}).<ref>{{cite journal|doi=10.1128/aem.00574-08|doi-access=free|title=Bacterial Community in the Crop of the Hoatzin, a Neotropical Folivorous Flying Bird|journal=Applied and Environmental Microbiology|volume=74|issue=19|pages=5905–5912|year=2008|last1=Godoy-Vitorino|first1=F.|last2=Ley|first2=R.E.|last3=Gao|first3=Z.|last4=Pei|first4=Z.|last5=Ortiz-Zuazaga|first5=H.|last6=Pericchi|first6=L.R.|last7=Garcia-Amado|first7=M.A.|last8=Michelangeli|first8=F.|last9=Blaser|first9=M.J.|last10=Gordon|first10=J.I.|last11=Domínguez-Bello|first11=M.G.|pmid=18689523|pmc=2565963|bibcode=2008ApEnM..74.5905G }}</ref>

Because of aromatic compounds in the leaves they consume, and the bacterial fermentation required to digest them,<ref name="Wright2009">{{cite journal|last1= Wright|first1=A.-D. G.|last2= Northwood|first2=K. S.|last3= Obispo|first3=N. E.|title= Rumen-like methanogens identified from the crop of the folivorous South American bird, the hoatzin (''Opisthocomus hoazin'')|journal= The ISME Journal|volume= 3|issue= 10|year= 2009|pages= 1120–1126|doi= 10.1038/ismej.2009.41|pmid=19387486|doi-access= free|bibcode=2009ISMEJ...3.1120W }}</ref><ref name="Godoy-Vitorino2011">{{cite journal|last1= Godoy-Vitorino|first1= F.|last2= Goldfarb|first2=K. C.|last3= Karaoz|first3= U.|last4= Leal|first4= S.|last5= Garcia-Amado|first5=M. A.|last6= Hugenholtz|first6= P.|last7= Tringe|first7=S. G.|last8= Brodie|first8=E. L.|last9= Dominguez-Bello|first9=M. G.|display-authors= 3|title= Comparative analyses of foregut and hindgut bacterial communities in hoatzins and cows|journal=The ISME Journal|volume= 6|issue= 3|year= 2011|pages= 531–541|doi= 10.1038/ismej.2011.131|pmid= 21938024|pmc= 3280141}}</ref> the birds have a disagreeable, [[manure]]-like odor and are only hunted by humans for food in times of dire need; local people also call it the "stinkbird" because of it.<ref name="DK"/> Much of the hoatzin's diet, including various types of ''[[Monstera]]'', ''[[Philodendron]]'' and other [[Araceae|aroids]], contains a high concentration of [[calcium oxalate crystals]], which, even in small amounts, can be greatly uncomfortable (and even dangerous) for humans to consume.

=== Breeding ===
Hoatzins are seasonal breeders, breeding during the [[Wet season|rainy season]], the exact timing of which varies across their range.<ref name ="HBW"/> Hoatzins are gregarious and nest in small [[bird colony|colonies]], laying two or three [[Egg (biology)|egg]]s in a stick nest in a tree hanging over water in seasonally flooded forests. The chicks are fed on regurgitated fermented food.

==Relationship with humans==
In Brazil, indigenous peoples sometimes collect the eggs for food, and the adults are occasionally hunted, but it is generally rare to consume mature birds, as hoatzin meat is reputed to have a bad taste.<ref name = "HBW"/><ref>{{Cite web|title=Hoatzin Opisthocomus hoazin|publisher=BirdLife International|url= http://www.birdlife.org/datazone/speciesfactsheet.php?id=1319|year=2004}}</ref> Its preferred habitats of forests and inland wetlands are threatened by [[deforestation of the Amazon rainforest|Amazonian deforestation]]. The hoatzin is believed to remain fairly common in a large part of its range, but its population is likely decreasing due to habitat loss.<ref name="iucn status 12 November 2021">{{cite iucn |author=BirdLife International |date=2016 |title=''Opisthocomus hoazin'' |volume=2016 |article-number=e.T22684428A93028795 |doi=10.2305/IUCN.UK.2016-3.RLTS.T22684428A93028795.en |access-date=12 November 2021}}</ref> The hoatzin is the national bird of [[Guyana]].<ref>{{cite web|url=http://www.guyana.org/Handbook/symbols.html|title=Guyana National Symbols|access-date=28 January 2014}}</ref>

==See also==
* The [[turaco]], a convergently evolved bird in the order [[Otidimorphae|Musophagiformes]], is a large-crested, arboreal, mainly herbivorous bird whose nestlings also use wing claws for climbing.<ref name="Parallel">{{cite journal|last1=Fain|first1=Matthew G.|last2=Houde|first2=Peter|year=2004|title=Parallel radiations in the primary clades of birds|journal=[[Evolution (journal)|Evolution]]|volume=58|issue=11|pages=2558–2573|doi=10.1554/04-235|url=http://biology-web.nmsu.edu/~houde/Parallel_radiations.pdf|pmid=15612298|s2cid=1296408|access-date=2016-07-08|archive-url=https://web.archive.org/web/20170709030601/http://biology-web.nmsu.edu/~houde/Parallel_radiations.pdf|archive-date=2017-07-09}}</ref>

==Notes==
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==References==
{{Reflist|30em}}<!-- {{cite journal|last=Cisneros-Heredia|first=Diego F. |year=2006|title=Notes on breeding, behaviour and distribution of some birds in Ecuador|journal=[[Bulletin of the British Ornithologists' Club]]|volume=126|issue=2|pages=153–164}} -->

== External links ==
{{Commons}}
* [[Cornell Lab of Ornithology]]: [https://www.birds.cornell.edu/homestudy/quiz/quiz-question-1 Mystery Birds: Hoatzin Adults and Young]. Retrieved 2008-JUN-16.
* [http://ibc.lynxeds.com/species/hoatzin-opisthocomus-hoazin Hoatzin videos, photos & sounds] on the Internet Bird Collection.

{{Opisthocomiformes|O.|state=collapsed}}
{{Taxonbar|from=Q188660}}
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[[Category:Opisthocomiformes]]
[[Category:Birds of the Guiana Shield]]
[[Category:Birds of the Amazon rainforest]]
[[Category:Extant Eocene first appearances]]
[[Category:Folivores]]
[[Category:Herbivorous vertebrates]]
[[Category:Herbivorous animals]]
[[Category:Birds described in 1776]]
[[Category:Birds of Brazil]]
[[Category:Taxa named by Philipp Ludwig Statius Müller]]