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Acorn Woodpeckers are known for being cooperative, if not slightly odd, birds. They live in commune-like family groups of up to a dozen individuals—some breed, others just help—while working together to collect and store acorns for food, defend their territory, and care for young. So it might be a little surprising to find out that these altruistic little birds often find themselves in vicious rumbles. The behavior is seldom seen, but recent footage caught by an ���Ϲ��� camera in California shows just how brutal the battles can be. 

In the video, woodpeckers dive-bomb one another from all directions, locking claws, pecking away, and tumbling around on the ground like they're in some avian version of UFC. One particular woodpecker seems to bear the brunt of the clash. As unchararacteristic as this behavior might seem, researchers call it a “power struggle,” and it’s considered a common part of the territorial fighting that happens between Acorn Woodpeckers.

The footage comes from the . Pete DeSimone, manager of the sanctuary, currently has installed in various locations throughout the property. DeSimone had heard of this type of behavior in the woodpeckers before, but he’d never seen it. “That we caught this event on camera was serendipitous,” he says

So what exactly is going on here? As it turns out, Acorn Woodpeckers are quite aggressive when it comes to defending their territories, and these so-called power struggles occur when breeding members of an existing group die, leaving vacancies that coalitions of woodpeckers from nearby groups will try to fill. When invading woodpeckers arrive is when the ruckus begins. The incoming breeding and non-breeding, or "helper," woodpeckers grapple with the group on the defense.

“When all of the breeders of one sex die, same-sex groups of helpers from nearby territories will fight each other for the right to breed at that territory,” says , a PhD candidate at Old Dominion University who studies the social behavior of Acorn Woodpeckers. “Rival coalitions will fight each other—think gang war—with the largest coalition winning . . . once the territory is secured, some of the [winning] coalition will return home, leaving their relatives to breed.”

Hagemeyer says Acorn Woodpecker groups are usually made up of up to seven closely related males—usually brothers or fathers and sons—and up to three closely related females—usually sisters or mothers and daughters—who breed with one another and together raise offspring in a communal nest. The offspring of the breeders become the helpers, and their job is to collect and store acorns and defend their granary. A group’s granary comprises the trees in which they've hidden their acorns, located in holes created by the birds' sharp beaks.

Evolutionarily speaking, power struggles are an instinctual behavior meant to maintain the Acorn Woodpeckers’ genetic diversity by preventing breeding between related individuals. And according to , the higher quality a granary, the more intense the power struggles will be. That’s because the quality of a granary is a limiting resource, with the number of acorns stored being positively associated with the woodpeckers’ overwinter survival and reproductive success. Two factors that are certainly worth fighting over.