For woodpeckers, acorns can be a matter of life and death. Acorn woodpeckers, which inhabit oak woodlands from Oregon to Mexico, will wage war against rival groups for days in pursuit of the acorns they need to survive the winter.
“We’ve seen birds with eyes gouged out, wings broken, bloody feathers and birds that fell to the ground fighting each other,” Sahas Barve, an ornithologist at the Smithsonian National Museum of Natural History, tells Priyanka Runwal of the New York Times. Barve is the lead author of new research, published this week in the journal Current Biology, that details the intricacies of these avian battle royales.
If this ferocious feathered spectacle sounds like it might be worth watching, you’re not alone. The researchers found that the territorial dust-ups are often attended by a crowd of non-violent onlookers, reports Kate Baggaley for Popular Science. These spectators will fly in from nearly two miles away, leaving their own territories unattended, just to spend an hour or so taking in the action.
The vicious fights, which can involve some 40 birds and feature continuous bouts lasting up to ten hours, are all in the name of gaining access to well-stocked “granaries,” which are dead trees that have been plugged full of thousands of acorns by the presiding woodpecker clan, according to Popular Science.
The granaries are controlled by social groups made up of as many as seven males that breed with between one and three females, per the paper. Strangely, these groups are usually composed of two unrelated sets of siblings—twin coalitions of brothers and sisters who mate with one another and vigorously defend the territory’s granaries against thieves.
The group is also bolstered by the presence of the offspring from prior years, called “helpers,” who typically hang around for five to six years to help their parents, aunts and uncles tend the nest. The helpers don’t breed in the group, and can’t reproduce until they find their own territories. As Runwal writes in the Times, “power struggles are thus a result of helpers striving to become breeders.”
When all the male or female members of a group controlling a choice chunk of habitat die off, it opens up an opportunity for coalitions of brothers or sisters acting as helpers in the area, which then descend on the area from far and wide and jockey for supremacy with one another to fill the vacancy. The coalitions are mostly made up of two to four birds and a dozen or more may show up to the fight, despite the fact that only one can win out.
"When you're approaching a big tree with a power struggle from far away, you'll first hear a lot of acorn woodpeckers calling very distinctly, and see birds flying around like crazy," says Barve in a statement. "When you get closer, you can see that there are a dozen or more coalitions of three or four birds fighting and posturing on branches. One group has to beat all the others to win a spot in the territory, which is really, really rare in animals—even in fantasy novels it usually boils down to one army against the other."
Using tiny radio transmitters attached to acorn woodpeckers in California’s Hasting’s Reserve, Barve and his team were able to study three power struggles in 2018 and 2019. Each battle involved coalitions of sisters fighting to fill the openings left by dead female woodpeckers, according to Popular Science. Until now, these avian conflicts have been too chaotic to study closely, but tiny harnesses strapped to the acorn-hoarding woodpeckers allowed the authors to uncover new and puzzling details.
Onlookers arrived within an hour of the fighting’s onset, according to the Times, and some of these passive observers spent as long as an hour of their day risking the security of their own acorns just to take in the violence. What exactly makes this expenditure of time and effort worth it to the birds on the sidelines isn’t clear.
“It seems like these power struggles are really important sources of social information,” Barve tells Popular Science. “Our best guess is [that] they get some juicy tidbits out of it.”
Damien Farine, an ecologist at the Max Planck Institute, tells the Times the study illustrates how tracking individual birds can illuminate how their societies function.
“With studies like this one, we’re starting to understand how populations are structured as an outcome of all its individuals’ behaviors,” Farine says.
This past summer, fires in California burned swaths of the oaky Hastings Reserve, reports Shawna Williams for the Scientist. Two large granaries that have been continuously restocked by successive generations of acorn woodpeckers for more than 50 years burned in the fire, and researchers are curious to see how the birds respond to the destruction. Will they make a new granary or wage a bloody war to take over another group’s territory?