Acorn woodpeckers wage days-long battles over vacant territories, radio
tag data show

Date:
September 7, 2020
Source:
Cell Press
Summary:
When acorn woodpeckers inhabiting high-quality territories
die, nearby birds begin a battle royal to win the vacant
spot. Researchers used radio tags to understand the immense
effort woodpecker warriors expend traveling to and fighting in
these dangerous battles. They also found spectator woodpeckers
go to great lengths to collect social information, coming from
kilometers around just to watch these chaotic power struggles.



FULL STORY ==========================================================================
When acorn woodpeckers inhabiting high-quality territories die, nearby
birds begin a battle royal to win the vacant spot. Researchers used radio
tags to understand the immense effort woodpecker warriors expend traveling
to and fighting in these dangerous battles. They also found spectator woodpeckers go to great lengths to collect social information, coming
from kilometers around just to watch these chaotic power struggles. The
work appears September 7 in the journal Current Biology.


========================================================================== "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 first author Sahas Barve, currently a postdoctoral fellow at the Smithsonian National Museum of
Natural History.

"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." The chaos of the battles
makes studying behavior using direct observation difficult. But Barve
and his team had an advantage: they used new radio telemetry technology
that allowed them to track the birds' locations down to the minute. With
radio tags, which "sit like a rock-climbing harness with a fanny pack
on the woodpecker's back," the researchers could learn how much time was
spent fighting at the power struggles and where the warriors came from.

Power struggles for co-breeding positions in oak trees with "granaries"
- - large acorn storage structures built by the birds consisting of
acorns stuffed into thousands of individual holes in the bark -- involve fighting coalitions formed by groups of non-breeding brothers or sisters
from neighboring territories. The radio tag data showed that some birds
return day after day and fight for ten hours at a time. "We didn't
think it could be that long because they have to be away from their
home territory," says Barve. "When do they eat? We still don't know."
The researchers hypothesized that woodpeckers would fight the hardest
for territories closest to their current home, but found that deciding
to fight may depend on more complex social cues as they recruit members
to join their coalition. "These birds often wait for years, and when
there's the right time and they have the right coalition size, they'll
go and give it their all to win a really good territory," he says.

The woodpeckers' complex social behavior also extends to the other
group that comes to power struggles: the spectators. "We never really
paid attention to them because we were always fixated on the birds that
were actually fighting," Barve says. "We often forget that there are
birds sitting on trees watching nearby." His team found that the biggest battles can attract more than 30 birds, or a third of all woodpeckers in
the area, with some traveling more than three kilometers to "come with
popcorn and watch the fight for the biggest mansion in the neighborhood."
The radio tag data also showed that the spectators spend up to an hour a
day watching the fights, despite many already having breeding position granaries of their own. For them, the benefits of social information
must outweigh the costs of leaving their home territory unattended for considerable amounts of time.

Acorn woodpeckers have tight social networks and know everyone's place
due to frequent travels to other territories. "If anything is disruptive
to that, or if anything weird happens, they want to go check it out,"
he says. "The spectators are probably as interested in the outcome
as the fighter is, although the warriors benefit more directly."
There's still a lot researchers don't know about the acorn woodpeckers'
complex social structures, but radio telemetry provides a glimpse into
their unique social behaviors. "They potentially have friendships,
and they probably have enemies," Barve says. "With our radio tag data,
we can tell when two birds are at the same place at the same time. The
next step is to try and understand how their social networks are shaped,
and how they vary across the year."

========================================================================== Story Source: Materials provided by Cell_Press. Note: Content may be
edited for style and length.


========================================================================== Journal Reference:
1. Sahas Barve, Ally S. Lahey, Rebecca M. Brunner, Walter D. Koenig,
Eric L.

Walters. Tracking the warriors and spectators of acorn woodpecker
wars.

Current Biology, 2020; 30 (17): R982 DOI: 10.1016/j.cub.2020.07.073 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2020/09/200907112342.htm

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