HANSEN • The red crossbills of the South Hills and the Albion Mountains are like no others in North America, and they’re in trouble.
There’s something remarkable, too, about these mountains’ lodgepole pines — something integral to the fate of this colorful bird with a highly specialized bill.
The tree and its big-billed seed predator are engaged in a contest of adaptation: pine cone defense versus the forager. That relationship soon could lead to species designation for the South Hills crossbill.
The scientist who described this bird as Loxia sinesciurus in 2009 returns each summer to the South Hills to study the bird’s ecology and population dynamics — and the cause for its massive decline.
Craig W. Benkman, an evolutionary ecologist at the University of Wyoming, is finishing an extensive genetic study and preparing to recommend a second time that the South Hills crossbill, now a call type of the red or common crossbill, be recognized as a distinct species.
“And if it is, it will be one of the most endangered bird species in North America,” he said.
The culprit? Climate change.
Scientists’ climate models predict a major reduction in the distribution of lodgepole pines — including their disappearance by the end of this century from the Idaho mountains that constitute the South Hills crossbill’s entire range.
“The outlook for this bird,” Benkman said, “looks really grim.”
• • •
On an early morning in June 2014, the cold campfire pit near a South Hills cabin becomes a research station.
This cabin southwest of the Diamondfield Jack Snowplay Area sits on a hillside, so posts support its front porch. Years ago, the cabin’s construction left something salty in the soil below the porch, and Benkman knows the crossbills will come for it.
“Birds come from all over to this spot,” he says. He found it in 2005 and started capturing crossbills under the porch in 2006.
Before 6 a.m., he and his field crew strung mist netting below two sides of the porch. Now the five wait quietly just uphill, where a plank resting on upturned campfire logs serves as their work table.
They’re waiting for crossbills to fly under the porch’s open south side, and they speak softly.
“We’ve got a few in the willows right now,” says Nate Behl, a UW graduate student carrying binoculars.
Some crossbills that fly under the porch will try to leave in another direction and be caught in the nets. When the researchers walk by, their movement will flush the rest.
Already, a couple of crossbills hang from the netting. “Maybe we should go for it,” Benkman says.
But Behl points out the crossbills he’s watching in nearby trees. “I’d give it another five minutes. We’ve got lots of birds around.”
When the researchers kneel at the nets, they work quickly and gently to free each crossbill.
“If they’re screaming, it’s very stressful for us,” Benkman says.
But these birds are quiet, and even the female with a string of netting caught in her leg band cooperates.
“This is going to be hard,” Benkman says, “because you don’t want to injure their wing. But fortunately she’s being very good.”
Soon each captured crossbill occupies a small cage. Three segmented boxes can hold eight birds each, visible through wire mesh on one side of the box and accessible through a flap on the other.
Before these crossbills can be released, the field crew must quickly take measurements and record the data they need to understand what’s happening to this bird population.
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The crossbills of the South Hills and the adjacent Albion Mountains are the largest ones north of Mexico and east of Newfoundland, the third largest of the 10 red crossbill types in the Western Hemisphere.
The secret is in the Rocky Mountain lodgepole pines that grow in these two southern Idaho mountain ranges — and the red squirrels that don’t.
In his scientific articles, Benkman describes an arms race between the lodgepole and the South Hills crossbill, with the tree evolving thicker scales on its cone and the bird evolving a larger bill to defeat that defense and reach the pine seeds that are its sole food source.
“This arms race has been escalating over the last 5- to 10,000 years,” Benkman said.
What does the red squirrel’s absence have to do with it?
A squirrel eats systematically, removing scales successively from the base of a cone to its distal end. It likes lots of seeds per cone. Where squirrels are present — throughout most of the range of the Rocky Mountain lodgepole pine — the tree invests in appropriate defense. Selection by red squirrels drives the tree’s cone structure: smaller cones with big bases, tapered tips and only 15 or so seeds.
But a few steps away from his makeshift research station in the South Hills, Benkman picks a cone to show how the lodgepole pine has developed differently here, where crossbills are the primary predators. The cone’s distal end is thick, with heavy scales. Inside might be 50 or 60 seeds.
A crossbill doesn’t care how many seeds are in each cone. To reach a single seed, it must pry apart the scales with its crossed mandibles and extract the seed with its tongue. Smaller-billed birds have a hard time surviving on these South Hills cones.
Here, more than 90 percent of individual lodgepole pine trees are serotinous, with seed release triggered by fire. As the cones weather gradually over years, their bonds weaken and the crossbills gain access to the seeds.
Heat waves, however, disastrously disrupt that food supply.
When summer temperatures exceed 90 degrees in the mountains, mimicking the effect of fire, the resin-sealed cones open and shed their seeds prematurely. That’s great for crossbills temporarily — in July or August — but leaves few seeds available in winter.
A former student of Benkman’s gathered pinecones of various ages to test at what temperatures they open. Benkman wonders: Do some individual lodgepole pines have higher temperature thresholds than others? Could planting those trees support South Hills crossbills longer? Maybe another century?
“But two centuries from now,” he said, “it’s going to be even hotter.”
• • •
The South Hills scenario is one of the world’s best examples of an evolutionary arms race, Benkman contends. And one of the best examples of an arms race causing divergence.
The resident crossbill population, he said, is genetically distinct, produces a different flight call than crossbills elsewhere in the Northwest and only rarely interbreeds with other call types that move into the South Hills yearly.
And he predicts the South Hills crossbill will be extinct by the end of the century.
Benkman first captured and studied South Hills crossbills in 1997. Before the heat waves of 2003 he observed a stable and abundant crossbill population from year to year. Since then, their decline has been dramatic.
The specialization that makes this bird population remarkable also makes it vulnerable. The South Hills crossbill population plummeted after 2003 solely because of decreased adult survival.
A 2011 paper published in the Journal of Animal Ecology, authored by Benkman and three other scientists, details research that pinned the blame on increasing spring and annual temperatures and the number of hot, dry days — rather than scaly-leg mites, West Nile virus or factors that affect nesting and breeding.
After the extraordinary heat of 2003, hot summers followed in 2006 and 2007. Lodgepole pinecones shed their seeds, and crossbill numbers suffered.
In the early years of Benkman’s South Hills research, 70 percent of adult birds survived from one year to the next. After 2003, that rate dropped to about 45 percent. Crossbill density — estimated through point counts at 72 spots scattered throughout the South Hills — declined by 80 percent between 2003 and 2011.
Both crossbill density and adult survival rose some in 2012, he said, but if adult survival stays below 70 percent the population will continue to decline.
If the South Hills crossbill gains recognition as a distinct species, Benkman said, it’s likely to become a poster child rather than a conservation project.
“There’s not much you can do about it, unless you can do something about climate change.”
• • •
On that June 2014 morning beside a South Hills cabin, Benkman’s research team has two telescopes set up near the campfire pit. To form survival estimates, they’re watching the trees for crossbills wearing tiny colored bands on their legs.
On the plank that serves as their research table, a binder lists the combinations of colored bands that — taken with other visible factors — identify individual birds at a distance.
One male with particularly red plumage and a big bill wears a red-and-white plastic band over a blue one. He’s “Captain America,” captured often enough that Behl memorized the number on his metal U.S. Fish and Wildlife Service leg band.
“He’s used to the drill,” says Shannon Carvey, a UW field technician.
For each crossbill lifted from the mesh-covered boxes, that drill includes banding — or examination of existing bands — and measurements of wing length and upper mandible depth and length. The team notes the condition of wing and tail feathers, evaluates abdominal fat, checks for parasitic leg mites and looks at females’ brood patches. Blowing at a female’s chest to spread her feathers, Behl explains the latter: A patch of skin on her chest was swollen, or vascularized, while she sat on her eggs. But her brood patch is wrinkly now, probably indicating her eggs have hatched.
Bill depth is particularly important because it differentiates South Hills crossbills from other types. With a small caliper, Behl takes multiple bill depth measurements for each bird; they’ll be averaged later. A nonresident crossbill moving through the South Hills will be carrying fat — another good way to spot call types from elsewhere in the Northwest, flying hundreds of miles between regions with large cone crops.
All of this must happen quickly. The researchers process juveniles first, because they’re more susceptible to stress, then females, so they can return to their nests.
Nathan Hough, a UW undergraduate, alerts Behl to a female with a smooth brooch patch in the next box. She’ll have priority.
The team is happy to discover that one of the male crossbills captured this morning hasn’t been caught since 2009. He’ll increase their survival estimates.
• • •
Ornithology’s “god squad,” as Benkman calls it, doesn’t yet recognize the South Hills crossbill as a distinct species.
The classification committee of the American Ornithologists’ Union evaluated and declined Benkman’s 2009 paper arguing for species recognition for the South Hills crossbill as Loxia sinesciurus, “without squirrels.” The vote was close.
By 2014 he had rectified the committee’s requests for more specimens and more data, adding 10 South Hills crossbills to the UW collections with call recordings and tissue samples for genetics. His team also analyzed several hundred million sequences of DNA across many North American crossbills.
“We can now show that the South Hills crossbill is genetically distinct from any red crossbill in North America,” Benkman said. “The evidence now is so compelling, once we have it peer reviewed and published I don’t think that there’s going to be any question about it.”
Field guides show just a red crossbill (Loxia curvirostra) and a white-winged crossbill (Loxia leucoptera). There’s a lot of variation within the reds, Benkman said, but the South Hills is the most genetically distinct by far. “If there’s any crossbill that deserves recognition at the species level, it’s the South Hills crossbill.”
All 10 call types of red crossbills look roughly the same, apart from variation in size and bill structure. But sonograms tell the difference.
In a 2012 article on eBird.org, Cornell Lab of Ornithology biologist and sound engineer Matt Young described the flight call of the South Hills crossbill, dubbed Type 9, as a distinctive, dry, “dip-dip.”
Type 2, the Ponderosa pine crossbill ranging continentwide in the U.S., produces a husky, deep “choowp-choowp.” Type 5, the lodgepole pine crossbill of the Western states, makes a twangy “clip-clip.” Both types move through the range of the South Hills crossbill each year.
“But they largely ignore each other. They treat each other as different species,” Benkman said. Females prefer mates of their own call types, and the types interbreed only occasionally.
• • •
For the 2014 capture team in the South Hills, the moment of each crossbill’s release is vital to determining whether it’s a Type 9, a resident of this mountain range. Cody Porter, a UW graduate student, opens his hands to release a bird while Carvey points a parabola, a dishlike device fitted with an omnidirectional microphone.
They’re hoping to record a flight call so a sonogram can confirm the type.
“If they don’t call, we don’t know, so we can’t use that data,” Benkman says. “Fortunately, most of them call.”
A year later, Benkman is back in the South Hills with a new team, and every few days he returns phone calls from a peak where he gets cell reception.
He has good news. The 2014 field work, he says, revealed crossbill density rising to reach half of its pre-2003 levels. The bird is coming back, and Benkman is more convinced than ever that the number of hot summer days in the past four years is the key to both decline and recovery.
He predicts adult survival will be strong in the crossbill data he’s collecting this summer, and the team should spot familiar combinations among the colored leg bands.
With luck, Captain America will be back.