It’s a plotline that George R.R. Martin of Game of Thrones fame might have concocted, and it’s all happening in the oak woodlands of Carmel Valley.
A closer look reveals the woodlands are not as serene as they seem at first glance. A fierce battle is brewing. Two sisters are seeking prime mating positions within their territory. Their mother becomes aware of her daughters’ plight and leaves her own territory to come to their aid. She risks her life fighting side-by-side with them against tenacious rival woodpeckers.
This intense skirmish at the Hastings Natural History Reservation is just one peek into the complex world of acorn woodpeckers. Mate-sharing, group sex, life-and-death battles, infanticide – researchers have seen it all over five decades of tracking the birds’ rare behaviors. More than 6,000 birds have been tracked in what is one of the longest coordinated vertebrate studies on Earth, according to Sahas Barve, a Hastings researcher.
And now, thanks to groundbreaking digital technology, researchers at this University of California outpost are able to track hundreds of the woodpeckers and their social interactions. The research, led by Dr. Eric Walters of Old Dominion University, is raising new questions about how the birds are able to communicate with each other – sometimes over a couple of miles – when a breeding position battle takes place. “No woodpecker checks into a power struggle on Facebook,” Barve says.
Researchers capture adult woodpeckers and equip them with solar-powered backpacks that include nano-tag radio trackers. Colored bands are attached to chicks’ legs for visual identification, and genetic testing is done to track family relationships over generations. The Hastings team is currently tracking around 300 birds within 60 groups.
“Even if we can’t watch the birds all the time, we know which territory it’s in,” Barve says. Researchers watch on a monitor tracking the birds’ precise locations, and can rush to the site of major skirmishes to witness events firsthand.
Another unique characteristic of the woodpeckers is what researchers call cooperative breeding. Multiple males and females breed together in a family group. Sometimes up to seven co-breeding males and four joint-nesting females will mate together. The males are closely related to one another – a father and sons, for example – and females are also close kin.
The cooperative parents are joined by nonbreeding “helpers,” offspring that choose to stay on and help raise subsequent broods of chicks. Helpers are being closely studied to determine what impact they have on the success of groups, Barve says, as well as why the birds choose not to breed. In addition to feeding and protecting chicks, helpers play key roles in building up what are called granaries – trees the woodpeckers drill holes into, then stuff with acorns in the fall. Barve says one group of birds has a huge granary with around 8,000 holes. Helpers also protect granaries from thieving squirrels and other species of birds. The survival of each woodpecker group depends on the nutrient-dense acorns to get them through winters.
Another role of helpers is serving as scouts to other territories looking for breeding vacancies – instances where a parent dies, creating room for a new mate. (Although promiscuous within groups, the birds are not incestuous, so if one mate dies a new mate from outside the family must take its place.)
These vacancies are rare and can turn into intense life-or-death battles, Barve says. In the case of a male vacancy, coalitions of males from other territories miles away will invade the territory and fight each other in violent tag-team matches, pecking at each other and plucking out feathers.
Helpers, however, choose to stay out of the fray and remain celibate, sometimes for up to seven years. (Most of the reserve’s woodpeckers live for five to eight years, but the record is 19; currently there’s a 17-year-old male and a 15-year-old female.) One theory: In territories where there’s plenty of food, there’s little motivation for helpers to shed any blood. Barve says there are cases where males will enter an area where there are no groups and start a new territory, but it’s a lot of work.
“If you strike out and start your own territory you have to store enough acorns to survive the winter,” Barve says. “There are tradeoffs to everything.”
As for the mother and her daughters, they won their power struggle. The daughters gained better breeding positions in the territory, and the mom lived to see another day back home. “Their social network is much more complex than we ever imagined,” Barve says.