Researchers are studying how climate change affects a bird that only lives in Idaho
In a stand of lodgepole pine trees, a songbird smaller than a robin dances on a branch.
“There’s one right up there laying around,” said Jennifer Boisvert, a research biologist with the Intermountain Bird Observatory at Boise State University, as she pointed her binoculars to a treetop.
“That’s what we’re always kind of listening for -- that chop-chop, chop-chop, chop-chop -- that’s the crossbill.”
She was looking for the Cassia Crossbill. It’s the only bird species endemic to Idaho, residing exclusively in two mountain ranges -- the South Hills, south of Twin Falls, and the Albion Mountains to the east.
With just a few thousand birds in total and only a small range to roam, researchers already believed the species could be heading toward extinction when, in 2020, a large wildfire burned through a significant portion of the lodgepole pine in its territory. Now, researchers like Boisvert are trying to assess the damage.
The dull red bird in the finch family gets its name from its thick, asymmetrical beak. The top part overlaps the bottom like one leg crossed over another. The peculiar beak wedges its way into the Rocky Mountain lodgepole pine’s serotinous cones, which are shut tight with resin, giving the bird special access to the seeds.
“Their bill works like pliers to kind of spread that open, and then they’re able to reach down in there with their tongue and pull that seed out for food,” Boisvert said.
In most areas with lodgepole pine trees, tree squirrels outcompete the crossbills to snag the seeds. But those rodents don’t live in the South Hills.
That has set up a unique adaptation battle between the birds and the trees, according to Dr. Craig Benkman, an evolutionary ecologist and professor at the University of Wyoming.
He’s the eminent expert on crossbills and returned to the South Hills annually after first noticing this unique species in 1996.
Every generation for thousands of years, Benkman said, the trees have grown cones with thicker scales, trying to fend off the birds.
“And as this occurs, it's going to favor a thicker and thicker bill, a bigger-billed bird,” he said.
Benkman labeled this dynamic a “coevolutionary arms race,” and said it’s what drove the Cassia Crossbill to become its own species, separate from other crossbills.
It wasn’t until 2017 that Benkman could prove this reproductive isolation to the American Ornithological Society with genetic data.
By that time, when the Cassia Crossbill was officially granted its own species status, Benkman knew it was already at risk. What makes the bird distinct also makes it vulnerable.
“It's only found in these two ranges,” he said. “It has no place else to go. There's no other option for them.”
In September 2020, the Badger Fire erupted in the South Hills. Pushed by ferocious winds, it ballooned to 90,000 acres. It burned through about a quarter of the lodgepole pine trees in the South Hills and Albion Mountains.
Boisvert pointed to one scorched forested patch near the top of a mountain.
“All of the trees are burned and have black fire scars around them,” she said. “And then when you look at the ground, you see mostly still just dirt and very little green vegetation after two summers of growth.”
Lodgepole pines are adapted to fire. The heat loosens their cones over time and sends their seeds to the ground to regrow the forest.
But intense fires, made more frequent with climate change, can mimic the effect of a heat wave. Several days of temperatures hotter than 90 degrees, Benkman’s research found, causes the pinecones to open prematurely.
Early cone-opening threatens the Cassia Crossbill’s long-term food supply. Instead of storing the seeds for decades, the cones drop them to the ground where other species can eat them. Fewer cones aging in the trees, Benkman said, means fewer crossbills in the future.
Plus, one study suggests lodgepole pines could stop growing in Idaho by the end of the century. Climate change makes the species’ decline almost inevitable, Benkman said.
“The question is, how much will that decline be?”
That’s a question Boisvert and her team with the Intermountain Bird Observatory are trying to get to the bottom of. For the past two years, in late summer and fall and for weeks at a time, researchers set out into the South Hills to do population surveys of the crossbills.
The group, based out of Boise State, also received some funding from the U.S. Fish and Wildlife Service, the Idaho Department of Fish and Game, the U.S. Forest Service and Tracy Aviary.
In the forested islands surrounded by sagebrush hills, they replicated some of Benkman’s earlier survey work, hiking and driving to more than 200 sites.
On one fall surveying day, Boisvert got an early 7 a.m. start. She hiked up a steep, wooded slope, not far from the highway, until she reached her GPS destination. She set up her clipboard and birding telescope.
“I’ll take a minute and kind of glance around at the trees and listen,” she said.
Then, she set a five-minute timer to count the crossbills she heard flying above or saw perched in trees.
“On that one, we had quite a few songbirds in here,” she said.
One crossbill flew overhead during the count. Then, it was time for another five-minute survey.
Boisvert also conducted a vegetation survey to get a sense of the habitat and the crossbills’ foraging patterns.
She counted the number of trees in a certain radius and recorded the species. She stretched her arms around the trunk of a 100-year-old lodgepole to note its diameter, and pointed her binoculars toward the canopy to count the cones branch by branch.
Luckily for the crossbill, the Badger Fire burned in a mosaic at higher elevations, so lots of tree stands were spared. Other parts of the forest didn't burn very hot and are now seeing regrowth.
A few months later, Boisvert and the Boise State team have analyzed the preliminary survey results.
Their population estimate showed about 500 fewer Cassia Crossbills in 2021 compared to the last time a comprehensive survey had been completed in 2016. That was one year after the Badger Fire. But by this fall, the population had declined by about half. They estimate about 3,000 of the crossbills remain.
This is not a surprise to Benkman, but he said it’s a stark reminder to pay attention to species in our backyards.
“Localized species that only occur in one mountain range, or one valley, or a small area -- those are the ones that are going to be most vulnerable with climate change,” he said.
Dr. Arne Mooers, a professor of biodiversity at Simon Fraser University in Canada, agreed with Benkman’s assessment, saying such species are “doubly-cursed.”
“A single extreme event can affect a large proportion of a small-range species,” Mooers said. “And small-ranged species are also often more specialized, meaning they just aren’t as good at rolling with the punches.”
Benkman and the Boise State team aren’t aware of an effort to submit a petition to list the Cassia Crossbill under the Endangered Species Act, though it could be a candidate because of its small population size and restricted range.
Their primary focus is using the data gathered to improve conservation efforts. Later this month, they’re meeting with the Sawtooth National Forest, the Idaho Department of Fish and Game, the U.S. Fish and Wildlife Service and the Shoshone-Bannock Tribes, to figure out how to manage the South Hills habitat to keep this rare bird around.
From Benkman’s research, they know the Cassia Crossbill population has dipped, and then rebounded, after a heat wave before. They’re hoping it can prevail again.
Clarification: This post has been updated to reflect Tracy Aviary also contributed funding to the research study and that the total number of survey sites is greater than 200.
Find reporter Rachel Cohen on Twitter @racheld_cohen
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