Tripod Tracks: When parrots take a walk, they're not copying anyone.

Lovebirds, small parrots with cheeky personalities, are popular pets. They wing 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 clamer up, and they cycle between left foot, right foot and a beak as if their mouths were another limb. In fact, a new analysis of the forces exerted by the climbing lovebirds reveals that this is precisely what they are doing.

Somehow, researchers wrote, 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 number of limbs for walking.

Dr. Granatosky speculates that parrots may have evolved this ability, like woodpeckers and nuthatches,  cannot hop up and down the trunk of the trees. [Veronique Greenwood]


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