Solar tracking gives scientists tools to follow small animals

( 21 June 2017; Photo: Hana Londoño Oikawa)

Keeping tabs on wild birds has long been a low-tech proposition. While radio collars and satellite tags became standard for tracking big mammals, binoculars and notebooks have remained critical for following most twittering, flittering birds.

The holdup has been battery weight. To bleep the signals that reveal an animal’s location, transmitter tags require power. But for birds that weigh no more than a few pennies, even a watch battery can be too much to bear. Add in the need for recapture every few days to replace those batteries, and the dream of automated animal tracking becomes a logistical nightmare.

Now, new, lightweight tracking tags are making small animal tracking feasible. During field testing at the NRS’s Hastings Natural History Reservation, it’s given scientists a whole new perspective on the acorn woodpeckers (Melanerpes formicivorus) they’ve watched for decades. The birds turn out to be a lot more sly and strategic than researchers had long believed.

“You’d think that after almost 50 years of study, we’d know all there was to know about acorn woodpeckers. But this technology is helping us to answer questions we couldn’t answer before,” says Eric Walters, a biology professor at Old Dominion University, Virginia.

Woodpecker families are weird

These scarlet-capped birds are head-scratchers, all right. For starters, their family structure is among the most peculiar in the avian world. Several related birds of one sex (often sisters), typically breed with an unrelated set of several related birds of the opposite sex (often brothers).

Another puzzler surrounds the life choices of young woodpeckers. Where most fledglings leave the nest to found their own families, young “helper” woodies may linger at home for a decade.

“The question we had is, why? There must be some benefit to staying at home,” Walters says. “Do they spend their time at home helping to feed their siblings and store acorns? Or are they like teenagers in the basement watching TV all day and doing nothing?”

Game of Granaries

Walters suspected the answer lies in a young woodpecker’s thirst for territory. Like the fictional nobles in “Game of Thrones,” birds will do anything to conquer their own kingdom. Their goal is to secure a good granary tree, which houses the acorn stash critical to a woodpecker family’s survival.

A massive granary tree affords a woodpecker family plenty of advantages. They’re more likely to survive harsh winters, raise surviving chicks, and ensure the triumph of their own bloodlines.

Good granaries are tough to come by. It takes an age to build one — the hole for one acorn might take ten minutes to excavate, and large granaries may hold tens of thousands of acorns. Competition to secure the biggest granaries and their surrounding territory is fierce.

Gang wars

A granary goes up for grabs only after all breeders of one sex in a family group have died or left. When a breeding spot opens up, birds from other families battle over the succession.

Successful combatants don’t show up alone. Instead, groups of siblings from other territories duke it out.

“It’s like a gang war,” says graduate student Natasha Hagemeyer, who is studying woodpecker dispersal at Hastings with Walters.

Clashes may last for days, with foes hunting one another through the trees, grappling in midair, falling to the ground, and hammering one another with their powerful bills.

Grievous injuries aren’t uncommon. “One had a toenail torn out and bled profusely. Another got some of its wing feathers ripped away; it disappeared within a week,” Hagemeyer says. But the potential spoils — the chance to breed — are worth any cost.

Waiting in the wings

When a rare vacancy does open up, aspiring replacements are ready. “When we caught a breeding bird to band, other birds thinking it was gone would move in and start challenging for that territory within ten minutes,” Walters says.

How neighboring birds figured this out was a mystery. “We almost never see them off territory,” Hagemeyer says.

Previous studies done with radio transmitters indicated young birds do go on longer forays off their own family’s domain. “But when you radio track on foot with handheld antennas, you can only follow one bird at a time. To follow a set of siblings, you need a one-to-one ratio of people and birds. It rapidly becomes infeasible. And if an animal wants to be sneaky, like going into an enemy’s territory, you can spook them, making tracking just about impossible,” she says.

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