https://www.scientificamerican.com/article/vicious-woodpecker-battles-draw-an-avian-audience/

The Americas’ western oak woodlands are fragmented into territories that are often fiercely contested—by groups of acorn woodpeckers. In each location, generations of the birds have transformed the oaks into granaries that store thousands of acorns. They nest in groups of breeding and nonbreeding members, which cooperatively raise chicks; when one of the breeding pair in a granary-rich area dies, rival teams of nonbreeding birds sweep in from surrounding territories to fight for a chance to fill it. These internecine struggles can be deadly, involve multiple coalitions of warriors and last for days. Scientists have studied the skirmishes for more than 50 years—but they only recently discovered other woodpeckers were keenly observing the battles, too.

Sahas Barve, an avian biologist at the Smithsonian National Museum of Natural History, was the lead author of a recent study that tracked this behavior and was published in Current Biology. He and his colleagues discovered the spectator phenomenon by fitting dozens of birds with ultralight solar-powered radio trackers and monitoring their positions. “Power struggles are so chaotic that you can’t really [visually] track the movements of any one animal,” Barve says.