Acorn woodpeckers fight real battles that last days to win the valuable territory that some of their species have left when they die, according to a new study published Tuesday that used radio frequency identification (RFID) tags to track the warrior birds.
The researchers also discovered that other individuals of the species flew from miles around to witness the clashes, gathering valuable social information about the power struggles.
The article was published in the journal Current Biology and edited by Sahas Barve, a postdoctoral fellow at the Smithsonian National Museum of Natural History.
« When you approach a large tree whose control is being contested from afar, you will first hear a lot of acorn woodpeckers calling in a very particular way and you will see birds flying around like crazy, » he said.
« If you get closer, you can see that there are a dozen or more coalitions of three or four birds fighting and perching on branches, » he added, which means that each battle may involve 40 or 50 woodpeckers.
« One group has to beat all the others to earn a place in the territory, which is really rare for animals. »
To make sense of the chaotic scenes, scientists placed RFID tags on the backs of 36 acorn woodpeckers in California.
Woodpeckers fought for control of « barns, » large acorn storage structures consisting of acorns tucked into thousands of individual holes in the tree’s bark.
Barns are valuable mating grounds, housing multiple breeding males and females and their young.
When a breeder dies, nearby groups of non-breeders form same-sex coalitions of sisters or brothers to try to win the vacant spot for their group’s breeder.
RFID tag data showed that some birds returned for several days to engage in hostile hand-to-hand collisions, incessant calls and intense fighting that were sometimes fatal.
« We didn’t think it could be that long because they have to be far from their home territory, » Barve said. « When do they eat? We still don’t know. »
The team had hypothesized that woodpeckers fought harder for territories closer to their current home, but found that more complex social forces were at play.
« These birds often wait for years, and when the time is right and they have the right coalition size, they will go and give it their all to win some really good territory, » Barve said.
The sophisticated social behavior was also observed in the fact that the battles drew huge crowds of up to 30 birds, some of which traveled up to three kilometers to be spectators. They would come up to an hour a day to watch the fights, even though many already had their own barns.
The researchers inferred that the benefits of social information are likely to outweigh the cost of leaving their homes unattended for so long.
Acorn woodpeckers live in close social networks and know the place they share due to their frequent trips to other territories.
« If something interrupts that, or if something strange happens, they want to go see it, » Barve said.
ia / dw / ll / gm