When acorn woodpeckers spot a potential new home, they join forces with their brothers and sisters for a battle royale. Up to a dozen teams of two or three fight to claim the spot for up to 10 hours per day for almost a week, with spectator birds flying in from kilometres away.
Sahas Barve at the Smithsonian National Museum of Natural History in Washington DC and his colleagues studied the movements of acorn woodpeckers in the Hastings Natural History Reservation in California.
They tracked the movements of birds using solar-powered radio tags during three separate battle events in 2018 and 2019.
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The battles, which lasted five consecutive days, were triggered by vacancies in oak trees containing 鈥� large storage structures that generations of birds have built, which consist of acorns stuffed into thousands of individual holes. The sibling teams hope to join the successful birds already living there.
鈥淚t鈥檚 like the most affluent family in the neighbourhood 鈥� when there鈥檚 a chance to become a breeder in those families, that when those big power struggles happen,鈥� says Barve.
The conflicts are triggered by new vacancies in both male and female birds, but the researchers only studied battles between teams of female acorn woodpeckers.
鈥淎t any given power struggle there can be upwards of 30 to 40 birds fighting,鈥� says Barve. The birds fight in sibling coalitions, with the winners needing to defeat all other rival teams. Some birds are seriously injured or killed.
鈥淭his sort of effort has not been documented in birds before,鈥� says Barve.
The conflicts drew spectators who lived farther away than the battling birds, some from more than 3 kilometres away. These spectator flew in daily and spent almost an hour watching the fight, despite many having their own acorn granaries.
Acorn woodpeckers have tight social networks and power struggle sites may provide an important source of social information, says Barve.
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