HANSEN — Expect to see a lot more birders with binoculars prowling the campgrounds of the South Hills and the Albion Mountains, listening for a colorful bird with a highly specialized bill.
Lodgepole pine forests of these two small mountain ranges are the only place to spot the Cassia crossbill (Loxia sinesciurus), newly recognized as Idaho’s sole endemic bird species.
The American Ornithological Society in April recognized the Cassia crossbill as warranting species status — a decision that put Idaho on the map for the American Birding Association’s big year route.
“It’s exciting. I think it’s a new place that all serious birders in the United States will now find their way to the South Hills of Idaho to add that species to their list,” Rexburg birder Darren Clark said in mid-August. “I think it could be a pretty cool thing.”
The phenomenon has begun.
Birders doing U.S. big years — competing to identify the most bird species in a single year — have been to the South Hills already this summer to see Loxia sinesciurus, said Clark, who has accumulated the biggest Idaho species list of all the birders with public eBird.org checklists.
HANSEN • The red crossbills of the South Hills and the Albion Mountains are like no others i…
A Hailey birder suggested Cassia crossbill T-shirts. One from Boise cautioned that outreach about the new species should include the correct pronunciation of “Cassia.” Magic Valley birders shared tips on the best campgrounds for locating the bird and offered to lead weekend trips. Another birder wondered whether local communities might embrace the bird and hatch a Cassia crossbill festival.
And others, with proprietary pride, assembled the short list of other states with their own endemic bird species — only Florida with the Florida scrub jay, California with the yellow-billed magpie and island scrub jay, and Hawaii with its long list. Alaska can’t entirely claim the McKay’s bunting, which has had a few sightings outside the state.
But the Cassia crossbill is all Idaho.
For that, you can thank the absence of red squirrels from the Rocky Mountain lodgepole pine forests of the South Hills and Albion Mountains. For that reason, there’s something remarkable about these pines, too.
“Crossbills are seed predators, so trees that deter foraging crossbills have a reproductive advantage,” says the new species page on Idahobirds.net. “Because crossbills occur in higher densities and consume more seeds in the South Hills and Albions than in lodgepole pine forests elsewhere, crossbills are especially important natural selective agents on the cones.”
In short, the tree and its big-billed seed predator are engaged in a contest of adaptation: pine cone defense versus the forager.
University of Wyoming evolutionary ecologist Craig W. Benkman described this bird as Loxia sinesciurus in 2009. In June 2015, when the Times-News published a “Big Story” report on his crossbill work in the South Hills, Benkman was finishing an extensive genetic study and preparing to recommend a second time that the Cassia crossbill — at that point considered 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 then.
In his scientific articles, Benkman describes the escalating arms race between the lodgepole and the Cassia crossbill, with the tree evolving thicker scales on its cone and the bird evolving a larger bill to defeat that defense and reach the seeds that are its sole food source.
What does the red squirrel’s absence have to do with that?
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 red squirrels are present — throughout most of the range of the Rocky Mountain lodgepole pine — selection by 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 in 2014, Benkman picked 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.
The result: Loxia sinesciurus.
“The new crossbill’s English name honors Cassia County, Idaho, which includes the complete known range of the species. North America already has birds named for cities (like Philadelphia vireo), states (Kentucky warbler) and countries (Canada goose), but this may be the first one named for a county,” Kenn Kaufman writes for Audubon.org.
The new endemic species isn’t the only crossbill in the South Hills. Birders in search of the Cassia crossbill might also encounter two call types of red crossbill, both of smaller average size than the Cassia crossbill.
“But because of extensive size overlap and no consistent plumage differences, one needs to rely on vocalizations,” Idahobirds.net advises.
Its page at idahobirds.net/birding-idaho/cassia-crossbill gives contact call recordings for all three birds. And it includes highly detailed advice on where and when to find the Cassia crossbill.
“Most crossbills are nomadic and therefore cannot be found reliably in any single area,” the page says. But the sedentary Cassia crossbill, which relies on the stable seed production of its arms race adversary, “is the only crossbill in North America that you can count on finding year-round, year after year, in the same forest.”
Clark hopes the celebrity of a new endemic species might help birders discover how good Idaho is for other birding, too — a place to spot your gray partridge, say. Or your tufted titmouse. Or your flammulated owl.