A hoatzin nestling balances on a twig.



Excerpt:

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.


https://www.science.org/doi/10.1126/sciadv.aat0787

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."

https://www.insidescience.org/news/baby-birds-use-wing-claws-climb-through-amazon#:~:text=Hoatzins%20are%20the%20only%20living,the%20water%20to%20escape%20predators.