International Wildlife

Crazy like a hoatzin

Crazy like a hoatzin – Venezuelan bird

Peter Zahler

To study a bird like this, you need to be a little balmy yourself

Yelling and clapping, we moved into the thick scrub, one eye on our quarry while the other searched for signs of feral boars and hornet nests. Dropping to our knees, we quick-crawled through the mud and twisted branches, our noisy progression matched by grunting alarm calls overhead. One hundred fifty feet and five minutes later, covered with muck and abrasions, we made one last howling charge into the scrub. From above, with wings frantically beating, a chicken-sized bird tumbled awkwardly to the ground. We pounced, and another hoatzin was banded and released.

The idea of running (or crawling) down a bird that is capable of flight may seem a little insane. However, nothing about the hoatzin is normal, from its looks to its behavior to its physiology to its taxonomic status. To do research on this bizarre bird, it helps to be at least a little crazy your- self.

Or a Big Cuckoo

The hoatzin’s scientific name, Opisthocomus hoazin, means “pheasant with a crest down its back.” A more accurate description of this demented-looking bird would be “punk rock chicken.” About the size of a good oven roaster, the hoatzin is a mess of chestnut-brown feathers with a bright blue face, red eyes and what appears to be a Mohawk haircut of feathers atop its head. The bird looks as if it would be at home leaping into a mosh pit at a rock club (and, in fact, its favored swampy habitat resembles the aftermath of a rainy outdoor concert).

However, the hoatzin is neither a pheasant nor a chicken. Its scientific name, combining Greek and the Central American language Nahuatl, only reflects the outlandish amalgam of physical and behavioral traits that have confused and baffled ornithologists since the bird was first described to science in 1776.

At times the hoatzin has been linked with the pheasants, the cuckoos, the pigeons, the cranes and even with that extinct avian archetype, the Archaeop- teryx. Only recently has research shed light on many aspects of its unlikely behavior, physiology and evolutionary lineage. For example, new laboratory work on hoatzin DNA has finally cleared up the deep taxonomic controversy. The studies have proven that hoatzins are indeed related most closely to cuckoos.

In all honesty, I have to say I’m not surprised.

Up the Creek With Hoatzin

A few years ago, I had the dubious honor of being enlisted to work on an ongo- ing research project on the hoatzin. The project’s location was on a cattle ranch named Hato Masaguaral on the flat and periodically flooded llanos, or plains, of central Venezuela.

Stuart Strahl, the project’s originator, spent two weeks acclimating me and my colleague David Lemmon to the world of the hoatzin. Since hoatzins nest in tree branches that hang over water, our work would involve a great deal of wading through the small creeks and swamps in the area.

Stuart, a large man, seemed happiest when slogging waist deep in the muck, hacking away at vines with his machete. When I tentatively asked him about any risk associated with the ubiquitous piranhas and alligatorlike caimans, he brusquely asserted that there was nothing to worry about. He’d never been bothered. However, the twinkle in his eyes made me especially wary of wiggling my fingers underwater during my stay at Masaguaral.

As it turned out, Stuart was right about piranhas and caimans, although step- ping on an underwater log that would swim off was a little disconcerting. However, Stuart did fail to warn us about attacks from feral boars, swarms of “killer” bees and hornets, and the periodic infestations of ticks and leeches.

Stuart was interested in two aspects of hoatzin life: their unusual dietary habits (and the peculiar and unique physiology associated with this diet) and their equally unusual cooperative lifestyle. In turn, our work consisted of four principal activities. The first was capturing and banding hoatzins, either by chasing and snatching the ones too clumsy to make it as far as a mist net, as described above, or plucking young birds from the branches before they could fly. Second, we waded or took a rubber dinghy out into the swamps and observed the identity and territorial boundaries of adult hoatzins. Third, we used a mirror attached to a long pole to observe the status of nests. Finally, we perched like clumsy hoatzins ourselves, high atop giant homemade towers, observing hoatzin group behavior.

What research like this has revealed about the hoatzin proves the maxim that even the most absurd object in nature is worthy of astonishment and even grudging respect.

The Flying Cow

The hoatzin’s nickname, “flying cow,” aptly describes the bird’s clumsiness in the air, but it also relates to the species’ unusual diet and physiological structures. Unlike most birds–which eat high-energy insects, seeds or fruits–the hoatzin is a folivore. It eats leaves. Leaves are notoriously dif- ficult to digest. They have tough, fibrous cell walls that make absorbing nutrients difficult. Leaves are also often loaded with unpleasant secondary chemical compounds such as tannins, terpenes and alkaloids that are designed to make the otherwise-defenseless leaves unpalatable or even poisonous.

To handle such a difficult diet, some animals, such as rabbits and rodents, pass food twice through the digestive system in a process called caprophagy. These animals produce a moist pellet that is then reingested to further the breakdown of the cell walls and neutralize the secondary compounds. Other animals, such as cows and horses, have evolved extra stomachs filled with bac- teria to help break down and detoxify the plant matter. The hoatzin uses this ruminant technique, but rather than grow an extra stomach, it has developed a unique method to deal with its leafy diet: foregut fermentation in the crop.

The crop, a chamber that is formed from the esophagus, is only found in birds. For some birds, the crop acts as a place to store food for young in the nest. The crop can also act as a place to “chew” one’s food, as birds lack teeth to grind food in the mouth. When a bird swallows pebbles and sand, the material ends up in the crop and is used to crush seeds and other hard food particles.

For the hoatzin, the crop has become not a pebble-grinder, but a bacterial fermentation tank. When a hoatzin swallows a leaf, the food’s first stop is the crop, where symbiotic bacteria break down the cell walls and make the nutritious cell contents available for digestion. The bacteria also appear to break down the plant’s defensive compounds that could otherwise interfere with digestion, or even poison the bird.

The hoatzin is the only bird, and one of the smallest organisms of any kind, to exhibit a ruminant-style digestive system. Not one to miss a trick, it also has an enzyme in its stomach that will break down and digest any bacteria that wander down from the crop. This means the hoatzin is getting a double dip–nutrients freed up from the leaves by the bacteria’s fermenting activity, and also nutrients from consumption of the bacteria themselves.

A Helping Claw

While wading through Masaguaral’s swamps performing nest checks, I learned about another of the hoatzin’s strange physical and behavioral traits. The mirror-on-a-pole technique we used accurately reflected the existence of the two or three eggs in each untidy nest. However, we had to be very careful about passing the mirror close to the nest. If the eggs had hatched, the sud- den appearance of the mirror could cause the naked, feeble nestlings to leap promptly over the nest edge (I assume this was not from the bird having seen its own reflection).

This apparent lust for suicide in young hoatzins would seem to be an evolutionary paradox; after all, a dead bird rarely breeds. However, it turns out that the hoatzin is more adept than it seems. Despite the nestlings’ help- less appearance, the little birds are world-class swimmers.

Each nest, positioned over the water, provides an excellent diving platform for a nervous hoatzin youth. Once in the water, the young bird quickly swims into the tangled foliage at the water’s edge. Although swamp water is full of danger, the chance of meeting a caiman during this swim is fairly low. However, if it doesn’t take the plunge, the baby hoatzin’s chance of escaping a predatory, weasel-like tayra or a marauding band of capuchin monkeys approaching the nest is zero.

When the coast is clear, the tiny hoatzin swimmer then uses a unique climbing adaptation–a pair of functional wing claws. These claws are usually absent in adults but provide a youngster with just the extra grip it needs to clamber back up the vegetation to the nest area. (These same claws are what led some people to suggest that the hoatzin was a “throwback” to some ancestral proto-bird, as wing claws are also found on the fossil remains of Archaeop- teryx.)

While the world of the baby hoatzin is fraught with peril, the nestlings are not left alone to face the dangers. Adult hoatzins, despite their absurd appearance and clumsy demeanor, make excellent and doting parents. Both mother and father brood the young and actively defend the nest, hooting and yelping at any potential predator or unwanted interloper. They both also feed the young a lovely leaf paste regurgitated from the crop, thus introducing the bacteria the young birds need to digest their leafy diet.

Or Like Generation X

As it turns out, the parents are not the only ones working to secure a baby hoatzin’s future. A variety of elder siblings, and occasionally complete strangers, will often help raise a nest of young. The birds may look like 80s punk rockers, but they seem to behave more like commune-dwelling hippies of the 60s.

Why should some adult hoatzins forego their own breeding to help raise another bird’s babies? The answer lies in a complicated blend of the hoatzin’s habitat and behavior.

During the breeding season, hoatzins are strongly territorial, and their ter- ritories include forest that borders on water to provide both nest sites and forage plants. Along a stream or around a swamp, all available “beach side” property may already be taken by established hoatzin pairs. In such situa- tions, the young bird often stays for a year or two in its parents’ territory. If a neighboring area opens up, the youngster can then try to grab it, or it can wait to inherit some part of the ancestral estate. The parents can use the extra help at the nest and an extra defender at the territory border, where squabbles can be intense.

The helper hoatzins are usually closely related to the new brood–in fact, they are essentially brothers and sisters. One scientific theory postulates that aiding close relatives increases a helper’s genetic contribution to the future almost as much as breeding on its own. Because a young hoatzin’s chances of successfully finding a territory and breeding on its own are slim to none, and because nests with helpers are more successful than those without, staying to help is evolution’s way of ensuring that some of a young hoatzin’s genetic material is passed on successfully.

Hangin’ With the Hoatzin

Although defending the nest or territory is an important part of a hoatzin’s daily routine, this activity takes only a tiny fraction of the day. Hoatzins spend most of their time either in short but frequent bouts of feeding or longer bouts of digestion. Even with the help of bacterial breakdown in the crop, the hoatzin must eat a great deal of its high-cellulose, leafy fare and then sit around for long periods for its digestive system to extract the nutrients the bird needs.

This means a hoatzin is often swollen with food and barely capable of flight. (Modifications of the sternum and pectoral girdle to fit the huge crop also reduce the hoatzin’s ability to fly, thus permitting seemingly deranged researchers to run them down.) This also means the bird is overloaded even when perched.

But again, the hoatzin has a clever answer. A large, rubbery callous the size and shape of a human thumb joint is positioned on the end of the hoatzin’s sternum, or breastbone. After a gluttonous bout of feeding, the bird squats down and rests its distended crop against a branch, using the callous as a tripodlike third leg.

I quickly grew to loathe this behavior. Part of my job was to identify individuals, using the unique combination of four colored legbands we put on every bird. Because the hoatzin’s crouch effectively hid the bird’s legs, identification was frequently impossible. Much of my time was spent standing waist deep in a swamp with a cloud of biting insects around my head, staring through binoculars at a fat and well-fed hoatzin, and uttering a variety of hoots and yelps designed to aggravate the hoatzin enough to get it to stand up so I could see the pattern of the color bands. The hoatzins found this behavior puzzling, and they would squat there, hooting and yelping back at me. Finally the whole experience would become too surreal for them and they would fly off, identity well concealed.

Six months is a long time to spend in a swamp if you’re not a hoatzin, but once I had time for reflection, I came to appreciate this ugly avian bovid. Despite its ridiculous appearance, and ungainly abilities, the hoatzin is superbly designed for its tropical home. It has developed a series of unique adaptations and behaviors that allow it to survive, and even thrive, in condi- tions that could make a bipedal primate cry.

COPYRIGHT 1997 National Wildlife Federation

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