There’s an Animal That Walks on Three Limbs. It’s a Parrot.

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Lovebirds, small parrots with vibrant rainbow plumage and cheeky personalities, are popular pets. They swing from ropes, cuddle with companions and race for treats in a waddling gait with all the urgency of toddlers who spot 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 clamber up, and they cycle between left foot, right foot and beak as if their mouths were another limb. In fact, a new analysis of the forces climbing lovebirds exert reveals that this is precisely what they are doing. 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 the way rock climbers use their arms.

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

“There is this very deep, deep set aspect of our biology that everything is bilateral” in much of the animal kingdom, he said. The situation makes it developmentally unlikely to grow an odd numbers of limbs for walking.

Some animals have developed workarounds. Kangaroos use their tails as a fifth limb when hopping slowly, pushing off from the ground with their posteriors the same way they push with their feet.

To see if parrots were using their beaks in a similar way, Dr. Granatosky and a graduate student, Melody Young, as well as their colleagues brought six rosy-faced lovebirds from a pet store into the lab. They had the birds climb up a surface that was fitted with a sensor to keep track of how much force they were exerting and in what directions. The scientists found that the propulsive force the birds applied through their beaks was similar to what they provided with their legs. What had started as a way to eat had transformed into a way to walk, with beaks as powerful as their limbs.

“For them to take their faces and integrate it into their stride cycle is pretty incredible,” said Ms. Young, who noted that the birds’ nervous systems would have had to change to fit beak movement into the rhythm of walking.

Dr. Granatosky speculates that parrots may have evolved this ability because they, like woodpeckers and nuthatches, cannot hop up and down the trunks of trees. Parrots alternate their legs when they walk, rather than pushing 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 could not provide to them.

How often parrots do this three-limbed walking in their daily lives is another question the researchers have. To get a sense of what role it plays in their behavior, Dr. Granatosky has dispatched students to make close observations of the green monk parakeets that live in the towering Gothic Revival-style gate of Green-Wood Cemetery in Brooklyn.

While the results have not yet been published, he hopes that 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 it.