---
title: "What the sources say about the Hoatzin (Stinkbird) | AnimalTracks.wiki"
description: "41 claims from the Hoatzin (Stinkbird) profile, each beside the words of the source that backs it."
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---

# What the sources say about the Hoatzin (Stinkbird) (*Opisthocomus hoazin*)

> 41 claims from the Hoatzin (Stinkbird) profile, each beside the words of the source that backs it. A "..." marks a cut in a quote and [brackets] an insertion.

Profile: [Hoatzin (Stinkbird)](https://animaltracks.wiki/animals/hoatzin/)

## Hoatzin. World Land Trust.

https://www.worldlandtrust.org/species/hoatzin/

| The page says | The source says |
| :--- | :--- |
| 62 to 70 centimetres from bill to tail and 700 to 900 grams | both are 62-70 cm from bill to tail and weigh 700-900g |
| It digests leaves mainly in an enlarged crop and lower oesophagus, where microbes ferment them | Unique amongst birds, their food is slowly digested mainly in their enlarged crop and lower oesophagus, a digestive process known as microbial foregut fermentation. |
| Since 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 | As 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') |
| That foregut is enormous, accounting for up to a quarter of the bird's weight | to accommodate a large foregut that accounts for up to 25% of their weight |
| To make room for it, the hoatzin's breastbone is much reduced, limiting it to weak flapping and gliding over short distances | the sternum is much reduced, limiting flight ability to weak flapping and gliding over short distances |
| It spends much of the hottest part of the day resting in trees while its food digests | Much of the hottest part of the day is spent resting in trees while their food digests |
| Its digestive system also earned it the nicknames "stink bird" and "stinking pheasant," after its strong, musky smell of fresh cow dung | The 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. |
| Hoatzins are highly social, often gathering in flocks of up to 100 in the dry season | This species is highly social, often occurring in flocks of up to 100 individuals in the dry season. |
| They 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 | Nesting 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 nest is a flat platform of sticks overhanging water, and the whole group incubates the two to four eggs for around a month | The 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 chicks carry claws on two digits of each wing, the alula and the major digit | One particularly notable feature is the wing claws on the alula and major digits possessed by the dark brown downy chicks. |
| When one threatens, a chick drops out of the nest into the water, swims, then climbs back into the vegetation using its wing claws | When 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 claws shrink as the chick grows | These are used for locomotion and reduce in size as the chick grows. |
| On their low-nutrient diet the chicks grow slowly | The chicks are fed regurgitated part-digested plant material by all the adults, growing slowly due to the low nutritional value of their diet. |
| they can't fly until they are around 60 days old | cannot fly until around 60 days old |
| Its 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 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. |
| Swamps, rivers, lakes and mangroves of the Amazon and Orinoco basins, mostly below 200 m elevation | Arboreal, 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 basins (note: 'mostly below 200 m' for 'mainly from around 5-200 m above sea level, though sometimes up to 500 m') |
| Young leaves and buds (about 82% of the diet), plus some flowers and fruit | new leaves and buds comprising 82% of their diet, flowers 10% and fruits 8% |
| It lives throughout the Amazon and Orinoco basins, in vegetation along and overhanging rivers, lakes, swamps and mangroves | 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 basins |

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

https://animaldiversity.org/accounts/Opisthocomus_hoazin/

| The page says | The source says |
| :--- | :--- |
| Their predators include monkeys, hawks and snakes | young Hoatzins have developed an unusual way of fleeing predators which include monkeys, hawks and snakes. |
| The hoatzin was described in 1776, and its classification has been debated ever since | Since 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) |
| a place on the bird family tree debated since it was described in 1776 | Since 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) |

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

https://doi.org/10.2305/iucn.uk.2024-2.rlts.t22684428a264086808.en

| The page says | The source says |
| :--- | :--- |
| Least Concern | IUCN Red List category: Least Concern (LC). |

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

https://neotropical.birds.cornell.edu/Species-Account/nb/species/hoatzi1

| The page says | The source says |
| :--- | :--- |
| It 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 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) |
| and are lost in the adult | The nestlings retain claws on their wings (lost in the adult), which they use to climb back to the nest. |
| it has been thought to be related to game birds, cuckoos, turacos, and more recently the group containing oilbirds and potoos | the 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 Hoatzin has defied easy classification, and its exact relationships with other birds is still not fully resolved | to date, the Hoatzin has defied easy classification, and its exact relationships with other birds is still not fully resolved |
| as a 2025 species account put it | Version: 2.0 - Published February 21, 2025 |
| Birds 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 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. |
| A 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 | 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. 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. |
| a bird that moves awkwardly and gives itself away with sounds of crashing through the vegetation, accompanied by loud calls | It moves awkwardly, however, and so gives itself away with sounds of crashing through the vegetation, accompanied by loud vocalizations (note: vibe.reason (#908)) |

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

https://doi.org/10.1126/science.245.4923.1236

| The page says | The source says |
| :--- | :--- |
| A 1989 study suggested the system may pay off through more efficient digestion, nutrients made by the microbes, and the detoxifying of plant compounds | Foregut 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 same study called hoatzins one of the smallest warm-blooded animals with this form of digestion | Hoatzins are one of the smallest endotherms with this form of digestion. (note: 'warm-blooded animals' for 'endotherms') |

## 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.

https://doi.org/10.1111/j.1096-3642.2006.00293.x

| The page says | The source says |
| :--- | :--- |
| A 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" | Recent 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 findings (note: 2007 is the reference's publication year (Livezey & Zusi 2007)) |

## 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.

https://doi.org/10.1073/pnas.2319696121

| The page says | The source says |
| :--- | :--- |
| A 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 | One 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') |
| such as penguins, albatrosses and pelicans | Most members of this group are seabirds and include such lineages as penguins, albatrosses, loons and pelicans. |

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

https://doi.org/10.1038/ismej.2009.41

| The page says | The source says |
| :--- | :--- |
| the hoatzin is the only bird known to digest by foregut fermentation | The hoatzin is the only known avian species with foregut fermentation. |
| Researchers who found that the methanogens in hoatzin crops are close relatives of known rumen methanogens | the hoatzin crop methanogen sequences formed sister groups with known rumen methanogens |
| called 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 crop microbial data presented here provide an excellent example of convergent evolution of foregut fermentation in the hoatzin, similar to that of ruminants. |

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

https://doi.org/10.1038/ismej.2011.131

| The page says | The source says |
| :--- | :--- |
| a system that evolved independently in birds and mammals | Regardless 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. |
| In what it does, not in how it got there. The hoatzin's crop fermentation evolved separately from the rumen of cows and other ruminants | Regardless 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. |