Old-Growth Forests May Help Songbirds Cope With Warming Climate

Each spring, songbirds migrate thousands of miles to breed in Oregon's Cascade Mountains. Deep in a forest, Oregon State University researcher Hankyu Kim feels he has gotten inside the head of one species, the hermit warbler.

"These birds are territorial in the breeding ground, they set up their territories, and they fight with each other to defend it," he says.

Armed with this knowledge, a nearly invisible net strung between two repurposed fishing poles, a lifelike plastic warbler decoy and a looped recording of birdcalls, Kim's trap is set.

His yellow hard hat matches the yellow head of the hermit warbler, which on cue flies down from the upper canopy of the trees to investigate the source of the song. Kim hides in the bushes, trying to follow the frantic bird with binoculars.

"When birds fly in, they hit the net and drop down into a pocket and lie down there like a hammock," Kim says.

And within just a few minutes, the hermit warbler takes the bait, flying across a small clearing and hitting the net.

It's another win for the decoy.

Kim and his colleagues are developing a new experiment, trying to track the movements of hermit warblers through the forest. Learning how they move could help explain how bird species are dealing with rising temperatures and climate change.

"We have these long-term population monitoring routes across the Northwest. And a surprising number of species are declining," says Oregon State professor Matt Betts. "Actually, more than about half of the species that live in a forest like this are in decline."

Rising temperatures can shrink where some birds can live and where they can find food.

For the hermit warbler, those declines are up to 4 percent each year.

Research by Oregon State's Betts and Sarah Frey found warblers declined in areas with young forests, including those replanted after clear-cut logging. But hermit warblers are doing better in other areas.

"In landscapes that had more older forest, their population declines were lowered, or even reversed, even though the climate has been warming," Frey says.

The Pacific Northwest has had a decades-long push to preserve its old-growth forests, and the warblers thrived in them. That suggests these forests somehow shielded them from the ill effects of rising temperatures.

The question is why, and that is where this new study comes in.

Kim and fellow Oregon State researcher Adam Hadley move the trapped hermit warbler's feathers aside and attach a tiny radio tag to its back using nontoxic glue (the kind used for fake eyelashes). Then they release the bird, and it flies away.

The next day is the true test.

Hadley and the others push into a dense stand of trees, armed with receivers that look like old-fashioned TV antennas.

"It's going away from us," Hadley says, holding the antenna over his head.

"We'll try to be as quiet as we can," says Betts, as branches snap underfoot.

They walk down a drainage though a 50-year-old tree plantation, a remnant of the logging past at the H.J. Andrews Experimental Forest. Then they cross into a grove of much older trees, some close to 300 years old.

Hadley explains that the temperatures can be different at various heights of a tree. "It's possible that when it's warmer, [songbirds] may be only using the bottom and more shady parts of the trees," he says. He guesses they may move up higher when it becomes cooler.

He says the complex layers and sheer biomass of old-growth keeps the temperature in these forests up to 5 degrees lower. But the researchers can't fully understand what's going on without knowing more about how the birds use the forests.

Hadley waves the antenna through the air trying to pinpoint the warbler's location. "I'm not getting the strongest signal at the top of the tree, seems to be a bit stronger in the midcanopy," he calls out to Betts.

He and the others will compare the hermit warblers' movements with temperature data they've also been gathering. They hope to get another step closer to understanding how this native songbird species might cope with the warming climate.

"I don't see it likely that hermit warblers will have air conditioning any time soon," Betts says.

Perhaps old-growth forests could be the next best thing.

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