There is an animal that walks on three limbs. It’s a parrot.

Lovebirds, tiny parrots with vibrant rainbow feathers and sassy personalities, are popular pets. They swing on ropes, cuddle with comrades and race for treats in a toddling hallway with all the urgency of toddlers seeing a cookie. But along with other parrots, they also do something strange: they use their faces to climb walls.

Give these birds a vertical surface to climb on, and they’ll cycle between left foot, right foot, and bill as if their mouth were another limb. In fact, a new analysis of the forces exerted by climbing lovebirds reveals that this is exactly what they do. Somehow, a team of scientists wrote in the journal Proceedings of the Royal Society B on Wednesday, the birds and perhaps other parrot species have repurposed the muscles in their necks and heads so they can walk on their beaks. , using them like mountaineers use their arms.

Climbing with a beak as a third limb is peculiar because third limbs are generally not something that life on Earth can produce, said Michael Granatosky, an assistant professor of anatomy at the New York Institute of Technology and an author of the new paper.

“There’s this very deep, ingrained aspect of our biology that everything is bilateral” across much of the animal kingdom, he said. The situation makes it developmentally unlikely that an odd number of limbs will grow for walking.

Some animals have developed solutions. Kangaroos use their tails as a fifth limb when they jump slowly, pushing their butts off the ground the same way they push with their feet.

To see if parrots used their beaks in a similar way, Dr. Granatosky and a graduate student, Melody Young, and their colleagues take six pink-faced lovebirds from a pet store to the lab. They got the birds to climb onto a surface equipped with a sensor to track how much force they were exerting and in which directions. The scientists found that the propulsive force the birds exerted through their beaks was similar to that through their legs. What had started as a way to eat had turned into a way to walk, with beaks as powerful as their limbs.

“It’s incredible for them to take their face and integrate it into their stride cycle,” said Ms. Young, who noted that the birds’ nervous system would have had to change to match beak movement to the rhythm of gait.

dr. Granatosky speculates that parrots developed this ability because, like woodpeckers and nuthatches, they cannot jump up and down the trunks of trees. Parrots alternate their legs when they walk, rather than push off with both legs at once. So when it came to the challenge of moving vertically, they had to come up with something different, something that created the third limb that developmental biology couldn’t provide them.

How often parrots take this three-pronged walk in their daily lives is another question the researchers have. To get a sense of the role it plays in their behavior, Dr. Granatosky students sent to closely observe the green monk parakeets that live in the towering Gothic Revival style gate of Green-Wood Cemetery in Brooklyn.

Though the results have not yet been published, he hopes the lovebirds and monk parakeets will help illuminate how parrots evolved such an unusual way of climbing and what changes they made to their bodies to do so.

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