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What the sources say

Hoatzin (Stinkbird)Opisthocomus hoazin

Every figure, record and mechanism on the Hoatzin (Stinkbird) profile was written from a source, and this is the working table behind it: 41 claims checked word for word against these sources. The left column is what the page says. The right column is the source's own words, copied, not paraphrased; a ... marks a cut and[brackets] an insertion. Open any source and check it yourself.

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Source
01

World Land Trust

Hoatzin. World Land Trust.

The page says62 to 70 centimetres from bill to tail and 700 to 900 grams
The source saysboth are 62-70 cm from bill to tail and weigh 700-900g
The page saysIt digests leaves mainly in an enlarged crop and lower oesophagus, where microbes ferment them
The source saysUnique amongst birds, their food is slowly digested mainly in their enlarged crop and lower oesophagus, a digestive process known as microbial foregut fermentation.
The page saysSince the hoatzin cannot chew, the walls inside have folds and ridges lined with tough, horny tissue that break leaves into pieces small enough for its resident bacteria to digest
The source saysAs they cannot chew, the muscular crop and lower oesophagus contain folds and ridges lined with cornified tissue to break down the leaves into pieces small enough for the bacteria to digest.Note: 'tough, horny tissue' for 'cornified tissue'
The page saysThat foregut is enormous, accounting for up to a quarter of the bird's weight
The source saysto accommodate a large foregut that accounts for up to 25% of their weight
The page saysTo make room for it, the hoatzin's breastbone is much reduced, limiting it to weak flapping and gliding over short distances
The source saysthe sternum is much reduced, limiting flight ability to weak flapping and gliding over short distances
The page saysIt spends much of the hottest part of the day resting in trees while its food digests
The source saysMuch of the hottest part of the day is spent resting in trees while their food digests
The page saysIts digestive system also earned it the nicknames "stink bird" and "stinking pheasant," after its strong, musky smell of fresh cow dung
The source saysThe Hoatzin's digestive system has also earned it the uncomplimentary nicknames of “stink bird” or “stinking Pheasant” after their strong, musky ‘fresh cow dung’ odour.
The page saysHoatzins are highly social, often gathering in flocks of up to 100 in the dry season
The source saysThis species is highly social, often occurring in flocks of up to 100 individuals in the dry season.
The page saysThey breed in the wet season, cooperatively: a monogamous pair and up to six helpers, usually males and mostly from earlier broods, defend a small territory next to water
The source saysNesting occurs in the wet season, the timing of which varies across their range. They are cooperative breeders consisting of a monogamous pair and up to six, usually male, adult or subadult helpers, mostly from previous broods, defending a small territory next to water.
The page saysThe nest is a flat platform of sticks overhanging water, and the whole group incubates the two to four eggs for around a month
The source saysThe female lays 2-4 eggs in an unlined, flat nest built of sticks overhanging water and the eggs are incubated by all group members for around 30-31 days.
The page saysThe chicks carry claws on two digits of each wing, the alula and the major digit
The source saysOne particularly notable feature is the wing claws on the alula and major digits possessed by the dark brown downy chicks.
The page saysWhen one threatens, a chick drops out of the nest into the water, swims, then climbs back into the vegetation using its wing claws
The source saysWhen threatened by a predator, the chicks fall into the water and swim before climbing back into the vegetation, aided by the claws on their wings.
The page saysThe claws shrink as the chick grows
The source saysThese are used for locomotion and reduce in size as the chick grows.
The page saysOn their low-nutrient diet the chicks grow slowly
The source saysThe chicks are fed regurgitated part-digested plant material by all the adults, growing slowly due to the low nutritional value of their diet.
The page saysthey can't fly until they are around 60 days old
The source sayscannot fly until around 60 days old
The page saysIts nickname comes from its digestive system, which World Land Trust says earned the hoatzin the uncomplimentary names "stink bird" and "stinking Pheasant" after its "strong, musky 'fresh cow dung' odour."
The source saysThe Hoatzin's digestive system has also earned it the uncomplimentary nicknames of “stink bird” or “stinking Pheasant” after their strong, musky ‘fresh cow dung’ odour.
The page saysSwamps, rivers, lakes and mangroves of the Amazon and Orinoco basins, mostly below 200 m elevation
The source saysArboreal, inhabiting vegetation, especially giant arums, along and overhanging rivers, streams, freshwater, brackish water and saltwater lakes, swamps, lagoons, and mangroves, mainly from around 5-200 m above sea level, though sometimes up to 500 m. The Hoatzin occurs throughout the Amazon and Orinoco basinsNote: 'mostly below 200 m' for 'mainly from around 5-200 m above sea level, though sometimes up to 500 m'
The page saysYoung leaves and buds (about 82% of the diet), plus some flowers and fruit
The source saysnew leaves and buds comprising 82% of their diet, flowers 10% and fruits 8%
The page saysIt lives throughout the Amazon and Orinoco basins, in vegetation along and overhanging rivers, lakes, swamps and mangroves
The source saysinhabiting vegetation, especially giant arums, along and overhanging rivers, streams, freshwater, brackish water and saltwater lakes, swamps, lagoons, and mangroves, mainly from around 5-200 m above sea level, though sometimes up to 500 m. The Hoatzin occurs throughout the Amazon and Orinoco basins

Source
02

Animal Diversity Web, University of Michigan

Opisthocomus hoazin (hoatzin). Animal Diversity Web, University of Michigan.

The page saysTheir predators include monkeys, hawks and snakes
The source saysyoung Hoatzins have developed an unusual way of fleeing predators which include monkeys, hawks and snakes.
The page saysThe hoatzin was described in 1776, and its classification has been debated ever since
The source saysSince Opisthocomus hoazin were first described in 1776, there has been a great deal of debate over their proper classification.Note: ADW (2001) says the debate has run since 1776 and 'is still being debated'; Birds of the World (2025) confirms it is 'still not fully resolved', so 'ever since' holds to date
The page saysa place on the bird family tree debated since it was described in 1776
The source saysSince Opisthocomus hoazin were first described in 1776, there has been a great deal of debate over their proper classification.Note: reviewer replaced the writer's computed '250-year fight' with the source's own framing: debate since the 1776 description

Source
03

The IUCN Red List of Threatened Species 2024

Opisthocomus hoazin. The IUCN Red List of Threatened Species 2024: e.T22684428A264086808.

The page saysLeast Concern
The source saysIUCN Red List category: Least Concern (LC).

Source
04

García-Amado & Billerman 2025

García-Amado & Billerman (2025). Hoatzin (Opisthocomus hoazin). Birds of the World, Cornell Lab of Ornithology.

The page saysIt is known as "a flying cow" because its crop digests leaves with the help of bacteria in a process akin to the fermentation used by ruminant mammals such as cows
The source saysThe Hoatzin is referred to as a flying cow: its diet primarily consists of young leaves and buds which are digested in the crop with the aid of bacteria and other microbes in a process akin to the fermentation process used by ruminant mammals.Note: 'because' renders the source's colon, which gives the diet and crop as the reason for the name
The page saysand are lost in the adult
The source saysThe nestlings retain claws on their wings (lost in the adult), which they use to climb back to the nest.
The page saysit has been thought to be related to game birds, cuckoos, turacos, and more recently the group containing oilbirds and potoos
The source saysthe Hoatzin has been thought to be related to Galliformes (especially the cracids and relatives), Cuculiformes (cuckoos and relatives), Musophagiformes (turacos and relatives), and more recently to the Strisores (oilbirds, potoos, and others)
The page saysthe Hoatzin has defied easy classification, and its exact relationships with other birds is still not fully resolved
The source saysto date, the Hoatzin has defied easy classification, and its exact relationships with other birds is still not fully resolved
The page saysas a 2025 species account put it
The source saysVersion: 2.0 - Published February 21, 2025
The page saysBirds of the World says the young can swim and may drop into the water when threatened, and the nestlings use their wing claws to climb back to the nest. The claws are "lost in the adult."
The source saysThe Hoatzin nests over water, and the young can swim and so may drop into the water when threatened. The nestlings retain claws on their wings (lost in the adult), which they use to climb back to the nest.
The page saysA crop that digests leaves with bacteria in a process akin to a ruminant's, and chicks with wing claws that climb back to the nest if they drop into water to escape predators
The source saysits diet primarily consists of young leaves and buds which are digested in the crop with the aid of bacteria and other microbes in a process akin to the fermentation process used by ruminant mammals. The Hoatzin nests over water, and the young can swim and so may drop into the water when threatened. The nestlings retain claws on their wings (lost in the adult), which they use to climb back to the nest.
The page saysa bird that moves awkwardly and gives itself away with sounds of crashing through the vegetation, accompanied by loud calls
The source saysIt moves awkwardly, however, and so gives itself away with sounds of crashing through the vegetation, accompanied by loud vocalizationsNote: vibe.reason (#908)

Source
05

Grajal et al. 1989

Grajal et al. (1989). Foregut Fermentation in the Hoatzin, a Neotropical Leaf-Eating Bird. Science.

The page saysA 1989 study suggested the system may pay off through more efficient digestion, nutrients made by the microbes, and the detoxifying of plant compounds
The source saysForegut fermentation in a flying bird may be explained by increased digestive efficiency by selection of highly fermentable and extremely patchy resources, coupled with microbial nutritional products and secondary compound detoxification.Note: 'plant compounds' for 'secondary compound'
The page saysThe same study called hoatzins one of the smallest warm-blooded animals with this form of digestion
The source saysHoatzins are one of the smallest endotherms with this form of digestion.Note: 'warm-blooded animals' for 'endotherms'

Source
06

Livezey & Zusi 2007

Livezey & Zusi (2007). Higher-order phylogeny of modern birds (Theropoda, Aves: Neornithes) based on comparative anatomy. II. Analysis and discussion. Zoological Journal of the Linnean Society.

The page saysA 2007 study of bird anatomy noted that "recent attempts to resolve the uncertainty of position of this monotypic lineage by molecular means have proven largely unsuccessful"
The source saysRecent attempts to resolve the uncertainty of position of this monotypic lineage by molecular means have proven largely unsuccessful, principally by mutual contradiction or ambiquity of findingsNote: 2007 is the reference's publication year (Livezey & Zusi 2007)

Source
07

Wu et al. 2024

Wu et al. (2024). Genomes, fossils, and the concurrent rise of modern birds and flowering plants in the Late Cretaceous. Proceedings of the National Academy of Sciences.

The page saysA 2024 genome-based tree places the "enigmatic hoatzin" in a lineage of aquatic groups that branch off one after another next to the core waterbirds
The source saysOne lineage, named Aequorlitornithes (3), contains a number of aquatic groups as well as the enigmatic hoatzin (Opisthocomiformes), as successive sister taxa to Aequornithes (core waterbirds).Note: 2024 is the reference's publication year (Wu et al. 2024); 'branch off one after another' for 'successive sister taxa'
The page sayssuch as penguins, albatrosses and pelicans
The source saysMost members of this group are seabirds and include such lineages as penguins, albatrosses, loons and pelicans.

Source
08

Wright, Northwood & Obispo 2009

Wright, Northwood & Obispo (2009). Rumen-like methanogens identified from the crop of the folivorous South American bird, the hoatzin (Opisthocomus hoazin). The ISME Journal.

The page saysthe hoatzin is the only bird known to digest by foregut fermentation
The source saysThe hoatzin is the only known avian species with foregut fermentation.
The page saysResearchers who found that the methanogens in hoatzin crops are close relatives of known rumen methanogens
The source saysthe hoatzin crop methanogen sequences formed sister groups with known rumen methanogens
The page sayscalled it "an excellent example of convergent evolution of foregut fermentation in the hoatzin, similar to that of ruminants": two separate lineages landing on the same solution independently
The source saysThe crop microbial data presented here provide an excellent example of convergent evolution of foregut fermentation in the hoatzin, similar to that of ruminants.

Source
09

Godoy-Vitorino et al. 2011

Godoy-Vitorino et al. (2011). Comparative analyses of foregut and hindgut bacterial communities in hoatzins and cows. The ISME Journal.

The page saysa system that evolved independently in birds and mammals
The source saysRegardless of the independent origin of foregut fermentation in birds and mammals, organ function has led to convergence of the microbial community structure in phylogenetically distant hosts.
The page saysIn what it does, not in how it got there. The hoatzin's crop fermentation evolved separately from the rumen of cows and other ruminants
The source saysRegardless of the independent origin of foregut fermentation in birds and mammals, organ function has led to convergence of the microbial community structure in phylogenetically distant hosts.

How the table was made: a writer drafted the profile from these sources and logged each claim with its quote, a reviewer with nothing but this table checked every sentence said no more than its quote, and a script confirmed each quote appears word for word in the source. The method is in the site'sAbout page.