The Baby Birds That Use Wing-Claws to Climb Through the Amazon

(Inside Science) -- A phoenix crossed with a dinosaur. That’s what the hoatzins looked like, with their lumpish bodies and spiked crests and blue faces. They squatted in the foliage at the edge of a lagoon in the Ecuadorian Amazon and made chuffing noises at our kayak. One lumbered aloft, made it to the next tree, and collapsed heavily in a spray of broken twigs.

And then I saw the chick. It was perched behind an adult, bobbing nervously. As the stubby wings splayed and wobbled for balance, I could just see where the wing’s leading edge split into a finger.

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The finger, I knew, was tipped in a claw, currently hidden in dark fuzz. Hoatzins are the only living birds with functional claws on their wings, a trait they lose as adults. The chicks use their claws to climb back into trees after dropping in the water to escape predators. But, save for a description and sketch published in 1888, there has been hardly any research on the wing-claws or how chicks use them -- until now.

A paper published last week in the journal Science Advances reveals the muscles and tendons that allow the claws to grasp. The researchers also found that hoatzin chicks crawl in a familiar way, alternating movements of front and rear limbs on opposite sides of the body. This is the walking gait used by animals such as lizards and dogs, but it has never before been documented in birds, said Anick Abourachid, a functional anatomist at the National Museum of Natural History in Paris, France, and first author of the study.

“It’s very strange for a bird,” said Abourachid. “There is no other bird that moves the wings in such a way, because all the other birds flap.”

A modern throwback

Hoatzins are often described as “living fossils,” and they certainly look like something that has remained unchanged since the Jurassic Period. Even the adults look odd, with small, spiky heads and bulging chests. (The chests contain enlarged chambers where bacteria ferment food, a system akin to that of cows and other ruminants.)

But the hoatzin’s lineage isn’t actually that old, having branched off relatively recently from other birds like hawks and doves. The hoatzin’s wing claws are probably a feature that was lost and then regained, rather than a relic left over from the age of dinosaurs, said Gerald Mayr, an ornithologist and curator at the Senckenberg Research Institute in Frankfurt, Germany, who was not involved in the study.

According to Mayr, there are significant differences between the skeletons of hoatzins and those of early birds like archaeopteryxes, which lived about 150 million years ago. The shoulder bones have different shapes, and the hoatzin lacks the archaeopteryx’s long bony tail. Thus, Mayr is skeptical that hoatzin chicks have much to tell us about the way the first birds moved.

In fact, Mayr suspects that our concept of what archaeopteryxes looked like may have been colored by artists who used the hoatzin’s plumage as inspiration. It’s not hard to find illustrations of archaeopteryxes sporting blue faces and feathery crests.

Nevertheless, when Abourachid and her colleagues conducted CT scans of a hoatzin embryo that was almost ready to hatch, they found that its claws and finger bones had proportions strikingly similar to those of an archaeopteryx. It’s easy to imagine that the ancient bird, like the modern one, used its prominent wing claws to climb.

Unique (but clumsy) movement

So just how do baby hoatzins use their claws? To answer this question, the researchers collected four hoatzin nestlings from the banks of the Cojedes River in Venezuela -- about 800 miles from the hoatzins I visited in Ecuador -- and placed the chicks in different situations.

Credit: Abourachid et al., Sci. Adv. 2019; 5 : eaat0787

In a tank of water, the chicks swam by moving both wings together in a pattern resembling the breast stroke. Such simultaneous wing motions are the norm for birds, although most employ them for flying rather than swimming.

When the hoatzin chicks were placed in trees, their limb movements were inconsistent, guided by the tangled shapes of branches, said Abourachid. Sometimes a chick used its head as a fifth limb, hooking branches with its beak.

The most interesting findings came when the researchers placed chicks on a slope covered with a towel. The bird’s progress up the slope was slow and halting, and it sometimes took a couple of tries before the wing-claws succeeded in grabbing the fabric. But the basic pattern was clear: right wing, left foot, left wing, right foot. Hoatzin chicks walk like four-legged animals, using their wings as front legs.

This is remarkable, said Abourachid, because synchronized flapping motions are deeply ingrained in bird nervous systems. When another team of researchers switched the spinal cord segments controlling the wings and legs of baby chickens, the normal limb coordination was reversed, so that the chicks kicked with both legs simultaneously. And yet somehow, baby hoatzins are able to use both wing movement patterns, taking advantage of their unique bodies to move in ways no other living bird can.

This sounds impressive in theory. In reality, baby hoatzin locomotion is a wobbly struggle, full of false starts and pinwheeling limbs. The researchers had to gently prod their chicks to keep them moving; the one I saw half-fell into the bowels of its tree. If archaeopteryxes did use the same strategies to climb, I have to imagine they were better at it.