November 2, 2022 | Jerry Bergman

What’s Up with Hoatzin?

The origin of the hoatzin still baffles evolutionists:
It is the strangest and most enigmatic bird species in the world



by Jerry Bergman, PhD

The hoatzin has defied attempts to determine its evolutionary history, prompting the use of DNA analysis. However, that approach has also failed to bridge the gap between this unique animal and its putative evolutionary ancestors. The problems are giving Darwinists nightmares.

The article reviewed below, rather than supporting evolution, actually supports creation. Ornithologist Joel Cracraft from the American Museum of Natural History has called the hoatzin “the strangest and most enigmatic species of bird in the world.”[1]

Why the most enigmatic bird?

Hoatzin nestling locomotion: acquisition of quadrupedal limb coordination in birds. 2019. From Wikimedia Commons.

Today the hoatzin (Opisthocomus hoazin) lives in the Amazon and Orinoco deltas of the South American rain forest and in a few lucky zoos. The bird is about the size of a chicken with short legs that hold up its stunningly colorful body. Several of its traits make it so unique that, in spite of years of effort, it could not be fit into any other bird family. Consequently, a whole new family had to be created for it— the Opisthocomidae.

Animals like the hoatzin, duckbilled platypus, armadillo, kiwi, glyptodon, and echidna are all problematic for evolution because each of these critters is a unique, one-of-a-kind species—too different from other animals to be placed in a large family. Thus, constructing evolutionary ancestral trees for them is very difficult. In Current Biology on October 24, Joel Cracraft described the taxonomists’ frustration with the hoatzin. During

the past half-century many morphological and genetic studies addressed the relationships of hoatzins with little to show for it. All manner of avian groups have been proposed as close relatives, from galliforms (chickens, turkeys) to cuckoos, pigeons, tinamous and a host of other orders of bird. The relationships of the hoatzin have been recalcitrant to multiple investigations, and even in our age of phylogenomics they remain so. With massive amounts of data and sophisticated phylogenetic algorithms, we still do not have a definitively clear idea about the closest living relatives of the hoatzin, as multiple genomic-scale analyses using different data sets and methods of analysis have been unable to find a strong consensus.[2]

Why the strangest bird?

The hoatzin has been called by various terms based on unusual traits, such as  “reptile bird” for the claws on the wings of the chicks, “skunk bird” for the foul smell due to its foregut fermentation of leaves, and “punk rock bird” due to its untidy crest of long, spiky, orange feathers.[3] More examples of frustration that taxonomists feel in trying to figure out its evolutionary pedigree include research that has

pointed to the shorebirds (order Charadriiformes) and the rails and cranes (Gruiformes) as the closest relatives of the Hoatzin, but other studies found hoatzins related to a huge group of orders having nothing to do with shorebirds and cranes — that is, no one knows for sure.[4]

Further reasons why it is so difficult to determine this bird’s phylogeny include a major

lack of agreement over which components of the genome are the most phylogenetically informative. Thus, studies use different samples of genomic data, combine them in different ways, and apply varying analytical models. … deciphering their interrelationships has been extremely difficult. This has been especially true of the Hoatzin because it stands by itself, being the only living representative of its lineage. Because of these factors, no matter what the phylogenetic result might be in any given study, statistical support for those relationships has been inconclusive.[5]

Hoatzins have another peculiarity from all other birds. For some unknown reason, they build their nests in trees over water. It’s not because they need to drink; ironically, they rarely drink water, because their diet of fruits, leaves, and flowers contains all of the water they require.[6] Living over water, though, creates a risk: if a chick inadvertently falls out of its nest, or jumps out due to being threatened, it falls into the water. When this happens, though, another unique design becomes apparent. The chick

can swim on the surface or under water, then go back to the tree and crawl up into it for safety. They accomplish this by another very strange adaptation for birds: the wings of young chicks possess two tiny claws at the ends of digits two and three that are used to lift themselves out of the water and move through the vegetation. These wing-claws are generally lost shortly after they can fly.[7]

The birds are powerful fliers near the waterways where they live. This requires comparatively large wings, to which are attached (in this case) brightly colored flight feathers. Their wings are not suited for long-distance flights, however. The hoatzin does not fly much more than from one tree to the next, mostly by crossing over from one branch to another.

Alimentary, my dear hoatzin

Another major difference between the hoatzin and other birds concerns its digestive system. A diet of leaves presents two problems: cellulose is hard to digest and contains less energy than other foods. The bird’s digestive system is unique, not only because it uses bacteria to digest cellulose, but because the hoatzin breaks down food while it is still in the lower portion of the esophagus in a receptacle called the crop. Other birds use their crop, or gizzard, to prepare food for digestion by grinding it up with small stones. The crop also serves to regulate the flow of food into the bird’s stomach. The hoatzin’s crop is about 50 times larger than its stomach to accommodate the large volume of leaves that it must consume daily to obtain the nutrients it needs to survive. The bird’s sternum (breastbone) is reduced in size because of the space required for this abnormally large crop. This feeding habit and digestive physiology is not used by any other bird, but it does resemble that of herbivorous mammals.

The hoatzin’s digestive system reverses the typical bird pattern. It uses a process called foregut fermentation.[8] Hoatzins are folivores – herbivores specializing in eating leaves. Mature leaves contain a high proportion of hard-to-digest cellulose, which provides less energy than most other food sources.[9] Consequently, folivorous animals like koalas have long digestive tracts and slow metabolisms. Many rely on symbiotic bacteria to release the nutrients from their leaf diet. A paper from 1993 in the Journal of Comparative Physiology noted that the hoatzin “is the only folivorous bird known to possess extensive fermentation in the crop by mixed bacterial populations.”[10]

The use of fermenting bacteria, common in herbivorous mammals but unique to the hoatzin, creates another challenge for evolutionary theory to explain. One suggestion made in the 1993 paper was that “evolution of foregut fermentation in mammals and birds might be a case of evolutionary convergence,” meaning that it somehow evolved only in the hoatzin genetic line and nowhere else among birds.[11] The idea of “convergent evolution” dodges scientific rigor by masquerading the explanation in terminology. Darwinians use the term “convergence” when attempting to account for the origin of a biological structure in one organism that resembles that in an unrelated organism. Note the following example of this fact-free tactic:

Sometimes natural selection nudges two organisms toward the same trait. Birds and bats independently evolved the ability to fly. Swifts and swallows each evolved into aerodynamic insectivores with nearly identical silhouettes, but traits such as their vocal organs and foot bones reveal that they are only distantly related. Because taxonomists often disagreed about things such as how to distinguish common ancestry [as distinct] from convergent evolution, the literature grew thick with conflicting trees, to the point that some twentieth-century biologists seemed ready to give up.[12]

Searching for evolutionary ancestry in genomes

Some evolutionists had hoped that genome sequencing would finally resolve phylogenetic trees for particularly difficult cases such as the hoatzin. The evolutionists’ goal was that the evolutionary tree of life would become

for biology what the periodic table was for chemistry—both a foundation and an emblem for the field. “The time will come I believe, though I shall not live to see it, when we shall have fairly true genealogical trees of each great kingdom of nature,” Darwin wrote to a friend. The rise of genome sequencing, at the turn of the twenty-first century, seemed to bring Darwin’s dream within reach. “It is now realistic to conceive of reconstructing the entire Tree of Life—eventually to include all of the living and extinct species,” … naturalist E. O. Wilson predicted that such a tree could unify biology. Its value to [evolution] …  would be incalculable; evolutionary trees have already deepened our understanding of sars-CoV-2.[13]

Josefin Stiller was a member of an international effort to sequence every bird genome. When Stiller joined the project, called B10K, her colleagues were combing through museums and laboratories to sample three hundred and sixty-three bird species, chosen carefully to represent the diversity of living birds. With help from four supercomputers in three different countries, Stiller and her colleagues began to compare each bird’s DNA to determine how they were related. The idea was that “once we sequence full genomes, we will be able to solve” the mystery of these birds’ evolutionary origins. Ben Crair, writing for The New Yorker, describes what happened next.

Unfortunately, early in the process, the researchers encountered the major evolutionary enigma called Opisthocomus hoazin. Soon their early optimism was shattered by what they found. One leading researcher involved in the study became “completely amazed by this bird,” and how the hoatzin could not fit anywhere in the evolutionary paradigm. The DNA, instead of helping to document evolution, did the opposite. One example:

Scientists had long assumed, for example, that daytime hunters such as hawks, eagles, and falcons all descended from a single bird of prey. But, in the genetic tree, hawks and eagles shared a branch with vultures, yet falcons turned out to be closer relatives of passerines and parrots. This meant that the peregrine falcon is more closely related to colorful macaws and tiny sparrows than to any hawk or eagle.[14]

In the end, the use of DNA research designed to help answer the mystery of the hoatzin

has not solved the mysteries of the hoatzin; it has deepened them. One 2014 analysis suggested that the bird’s closest living relatives are cranes and shorebirds such as gulls and plovers. Another, in 2020, concluded that this clumsy flier is a sister species to a group that includes tiny, hovering hummingbirds and high-speed swifts. “Frankly, there is no one in the world who knows what hoatzins are,” Cracraft …  said. The hoatzin may be more than a missing piece of the evolutionary puzzle. It may be a sphinx with a riddle that many biologists are reluctant to consider: What if the pattern of evolution is not actually a tree [showing a progression as evolution predicts]?[15]


The hoatzin’s morphology and specific traits do not fit into the evolutionary phylogeny for birds. When anatomical traits failed to fit the hoatzin into a phylogenetic tree, DNA analysis was tried. Unfortunately, the hoatzin genome also failed to identify any close evolutionary relationships to other birds.[16]

The example highlighted here is only one case (albeit a blatant example) of a general trend in genomic studies of evolution. DNA often fails to reveal common ancestry between distant biological groups—and sometimes between close families. Rather, the bulk of morphological and genomic evidence supports the view of separate creation of a large variety of creatures within family groups that reproduce after their kind. Such a view, first taught in Genesis, still holds.


[1] Cracraft, Joel. 2022. The Hoatzin. Current Biology 32(20): R1068-R1069, p. R1068, October 24.

[2] Cracraft, Joel. 2022. The Hoatzin. Current Biology 32(20): R1068-R1069, p. R1068, October 24.

[3] Crampton, Linda. 2022. 40 Hoatzin or Stinkbird Facts: A Strange and Unique Animal. Owlcation, May 27.

[4] Cracraft, 2022, R1068.

[5] Cracraft, 2022, R1068.

[6] Dominguez-Bello, M.G., et al., 1994. Ecology of the Folivorous Hoatzin. The Auk 111(3): 643-651, July 1.

[7] Cracraft, 2022, R1068.

[8] Dominguez-Bello, M.G., et al. 1993. Evolutionary significance of foregut fermentation in the hoatzin (Opisthocomus hoazin; Aves: Opisthocomidae). Journal of Comparative Physiology B 163(7): 594–601, December; doi:10.1007/BF00302119.

[9] Cork, S.J., and A.C.I. Warner. 1983. The passage of digesta markers through the gut of a folivorous marsupial, the koala Phascolarctos cinereus. Journal of Comparative Physiology 152:43–51.

[10] Dominguez-Bello, et al., 1993.

[11] Dominguez-Bello, et al., 1994.

[12] Crair, Ben. 2022. The Bizarre Bird That’s Breaking the Tree of Life. Darwin thought that family trees could explain evolution. The hoatzin suggests otherwise. Real Clear Science, July 19.

[13] Crair, 2022.

[14] Crair, 2022.

[15] Crair, 2022.

[16] Crair, Ben. 2022. “The Bizarre Bird That’s Breaking the Tree of Life.” The New Yorker (July 15, 2022); elements/the-bizarre-bird-thats-breaking-the- tree-of-life/.

Dr. Jerry Bergman has taught biology, genetics, chemistry, biochemistry, anthropology, geology, and microbiology for over 40 years at several colleges and universities including Bowling Green State University, Medical College of Ohio where he was a research associate in experimental pathology, and The University of Toledo. He is a graduate of the Medical College of Ohio, Wayne State University in Detroit, the University of Toledo, and Bowling Green State University. He has over 1,300 publications in 12 languages and 40 books and monographs. His books and textbooks that include chapters that he authored are in over 1,500 college libraries in 27 countries. So far over 80,000 copies of the 40 books and monographs that he has authored or co-authored are in print. For more articles by Dr Bergman, see his Author Profile.

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