The West’s Newest Bird Species has a Beak Like a Crowbar
A recently discovered species of crossbill already faces extinction.
At 6:30 one morning in early July, Craig Benkman, a University of Wyoming ecologist, began to stalk red crossbills in the South Hills of Idaho. We were between Twin Falls and the Nevada border, outside a cabin tucked into a forest near a minor ski hill, Magic Mountain. He and several of his graduate students had quietly strung ornithology’s signature ploy, a diaphanous mist-net, between two metal poles. It hung nearly invisible, low to the ground, below two lodgepole pines. Several of these sanguine birds — a large finch with a beak that looks curiously off-kilter, one mandible overlapping the other — were resting in the grass by a salt lick. Crossbills feed exclusively on the seeds of conifer cones, and must supplement their diet with sodium. “It’s like how, in the Sierra, marmots chew on your sweaty boots,” Benkman explained. Typically the birds might swallow a little clay from the roots of a fallen tree for salt, but here cabin owners had arranged mineral blocks in the grass to lure moose and deer nearer to their windows. Benkman peered through his binoculars at a male sitting with several others beside the net, all of them looking up warily with glossy black eyes. The male’s domed head, breast and rump were the crimson of an heirloom tomato, flecked with orange, with yellow ochre. “Oh,” Benkman said, in a rising whisper. “An unbanded one.”
Six feet four, thin and bespectacled, Benkman has a hushed voice and a sense of humor as dry and pleasant as the sagebrush- and wildflower-spotted South Hills. He is an old-school naturalist and an intuitive, holistic thinker, something increasingly rare in this era of computational biology and specialization. Walking the South Hills, he refers to minute flowers by their Linnaean names and muses about the West’s big ecological picture as his jeans swish through the fragrant scrub. For more than 30 years, he has studied crossbills across North America, as well as in aviaries, and for the past 20 summers, he has visited this remote mountain range on the edge of the Great Basin, where he and his students aim to capture as many crossbills as possible and mark them with colored leg bands. When he received the prestigious E.O. Wilson Award from the American Society of Naturalists in 2014, the judges remarked that his crossbill work “rivals long-term field studies of the Galapagos finches.”
The mist-net swayed, reminding the nervous birds in the grass of its shadowy presence. “I wish the wind would just stop,” Benkman said, as if to the wind itself. “This is where I think you need to be aggressive and try to spook them. You take off your shirt and throw it — make it look like a goshawk.” He started to peel the plaid shirt from his long pale torso, but I offered up the green fleece I’d shed already. Benkman tied it into an aerodynamic knot, a mock raptor. “For an unbanded bird?” he said. “You can’t just stand idly.” He began to slink circuitously toward the salt lick.
When crossbills are feeding happily, all you hear is the soft rain of translucent husks slipping through needled branches. Then one might let forth a kip. For the red crossbill, Loxia curvirostra (“curved nose”), this call note is the key to another peculiarity: Unlike most birds, it comes in a range of body and beak sizes, a smooth, vermilion continuum from relatively delicate to bulky. This diversity confounded birders for more than a century because our very notion of a species is predicated on interbreeding, which tends to generate uniformity. But as it turns out, each relative size has its own inflected kip. There are 10 red crossbill “call types” in North America, and the birds within each type seem to mate and flock among themselves. Yet an individual bird can’t be “typed” reliably by measurement or ear. Only a spectrograph of the call — a visualization that recalls the brushstrokes of Japanese calligraphy — can diagnose a red crossbill definitively. The kip variants help the birds identify and stick with other red crossbills that have similar-sized bills, since when everyone has the same “tool,” and thus the same standards, a flock can better judge the caliber of cones. They watch each other and broadcast information about seed abundance by voice. “It’s as if they’re calling, ‘I’m doing poorly — how are you doing?’ ” Benkman told me. “If nobody else calls, ‘Well, I got a bad branch, a bad cone,’ then they keep foraging and quiet up. But if they all call, ‘I’m doing poorly, you’re doing poorly,’ then it’s a bad tree, and they move on.” But these cues are only reliable if everyone is similarly equipped.
As a young Loxia-phile, Benkman made his name, in part, when he hypothesized that red crossbill types might be specialized to feed on conifers that hold their seeds year round, making them stable resources. To test this suspicion, he performed a now-classic Darwinian experiment: In aviaries, he measured crossbills’ feeding rates on the Northwest’s “key conifers,” and he found that each call type in the region just about matched the perfect bill depth for exploiting one species of cone. Large bills are ideal for ample, bony cones like ponderosa pine; lesser bills for tiny, soft cones like hemlock. As Benkman later put it in a paper, “Their bills are as finely adapted to a cone as a key is to a lock.” Coupled with the call type phenomenon, this revelation has encouraged ornithologists to speculate that some or all of the call types might be “cryptic” species hidden in our midst, each filling a different coniferous niche. Parsing the red crossbill, and potentially paring it into species, has long been considered one of ornithology’s most tantalizing problems.
Like a cat that had slipped its bell, Benkman crept hunched in a wide arc through the tall grass of the South Hills’ lodgepole pine forest. Slowly, he raised the catapult of his arm — and then the polyester goshawk took flight, flushing three birds like flares into the net’s diaphanous pockets. Benkman and his students rushed forward and began to untangle them, pinching the birds’ legs between their fingers, using their thumbs to tease the thread-thin cord over the birds’ shoulders. Cupped in your hand, a crossbill pulses like a winged heart. They would stash the birds in a wooden box with eight compartments that Benkman custom-built 30 years ago. Each would be measured with a ruler and digital calipers, weighed, and finally recorded as it flew off with a revealing kip. But now, as the birds hung in the net, so incendiary in their plumage and sometimes their cries, they were in limbo in more ways than one: Benkman and his colleagues believed that they should be named a new species, the first bird truly discovered in the Western United States since the Gunnison sage grouse in 2000, and only the second in the continental U.S. since 1939. They had also discovered that this bird is likely to disappear.
But there was one spot in North America where a sedentary, or resident, population of crossbills was said to live: Newfoundland. Benkman visited there in 1988 hoping to find this large-beaked bird, call type 8, known locally as the “spruce mope” for its treetop nonchalance. When he arrived, though, he discovered that the mope had grown as scarce as rumored, possibly to the point of near extinction, which was odd because the island’s spruce forest looked healthy and just like the kind that crossbills relished on the mainland. Finally, Benkman told me, he had an epiphany: Maybe red squirrels were to blame. They didn’t exist in Newfoundland until 1963, when humans released them as prey for the native pine marten, an animal in the weasel family that had been over-trapped for its lustrous coat. This plan was a serious dud: Red squirrels — also known, onomatopoetically, as chickaree — are arboreal, and martens mostly hunt along the forest floor. Because Newfoundland’s spruce had evolved no defenses against the rodents, they feasted unchecked across the island to the mope’s detriment, chewing apart cones from base to tip and dropping scales to the ground like shingles from an old torn-up roof.
As Benkman’s peregrinations in search of crossbills continued, he resolved to keep an eye out for other places without squirrels, including isolated mountain ranges, “islands in the sky.” In 1993, he visited the Cypress Hills in Saskatchewan, said to have no squirrels and another large, sedentary crossbill, but chickaree had been introduced there, as well, and the crossbill had vanished. Yet the lodgepole cones were strangely massive, a glaring outlier in North America. Where squirrels are found, they dictate the shape of cones by harvesting those that are easiest to open, with lots of seeds. Gradually, the forest’s cones will shift in size, on average, becoming smaller or larger — for lodgepole pine, cones become smaller and denser, but relatively wider at the base, making them harder to gnaw. They will also contain fewer seeds: less bang for a squirrel’s buck. Benkman thought that, in the absence of squirrels, the Cypress Hills trees had relaxed these defenses and devoted more energy to seeds, bearing along a heftier endemic crossbill as a byproduct, though the bird had now been outcompeted and lost forever.
Benkman returned to the Cypress Hills to study those invasive squirrels in 1995. “We were wandering around,” he recalls, “and went to a fire lookout and, from the top, I saw the Sweetgrass Hills.” He and a student contemplated these isolated buttes on the prairie horizon, about 60 miles away. If the cones in the Cypress Hills had enlarged sans squirrels, what about those nearby, in northern Montana? When Benkman drove to the range, it was void of squirrels, but the lodgepole cones were a normal size. “It was a revelation,” says Benkman. “It wasn’t just relaxation of squirrel selection in the Cypress Hills. Crossbills had played a role.” They must have coevolved with the pines — something Benkman had never quite fathomed. As crossbills pried open and so “selected against” smaller, easier cones, the forest’s average cone had gradually, unwittingly, ballooned in self-defense. The bird’s bill kept pace in an “arms race.” Maybe coevolution explained the existence of Newfoundland’s large resident bird, too.
Benkman scrutinized maps and explored other ranges in the Interior West, until in 1996, finally he came to the South Hills, a volcanic plateau with a patchwork of lodgepole pine on its heights. When Benkman drove into this rolling range marooned in the sagebrush and circular, irrigated fields of the Snake River Plain near Twin Falls — a region known as Magic Valley ever since twin dams colored it a magnificent, unlikely green a century ago — he discovered that, indeed, there were no squirrels. But that wasn’t all: Crossbills were everywhere. “And they sounded a little different,” Benkman remembers, softer around the edges, more “burry.” Benkman had chanced upon a rare sedentary population, like the mope in Newfoundland. It was a new red crossbill call type, the ninth in North America, and it had the third-largest bill among them.
The South Hills are known as “Magic Valley’s Playground,” and, throughout the summer, ATVs roar up and down its dirt roads, raising a ruckus and the dust, transforming its otherwise tranquil atmosphere. But each morning you awake early in your tent to a liquid fortissimo that implies one thing: no bushy tails. Squirrels also prey on eggs. Without them, two times as many birds are found in the South Hills than in nearly identical, but squirrely habitat in the Northern Rockies: western tanagers, flycatchers, house wrens, warblers, warblers, warblers. In turn, the range has one of the highest known densities of breeding northern goshawks — the scourge of smaller birds — in the world. It’s an ornithological oasis, and a crossbill-topia. There are commonly dozens nearby, kip-ing, hanging upside down from cones — classic crossbill behavior — and descending to springs and salty ground, where Benkman first netted them out of his pickup. Finding a sedentary call type in the South Hills was a godsend; Benkman could give up his peripatetic ways, at least partially, and settle down.
Another major ecological force helps explain this unique population: fire. Nearly all of the lodgepole pine in the South Hills is serotinous. Their cones’ scales are held tightly shut by a resinous bond that releases only in extreme heat. This strategy has its advantages in the blaze-prone Interior West. But where squirrels rule, it’s a slight handicap, because it merely gives the zealous rodents a longer opportunity to harvest cones. In the Eastern Rockies, only a third of the lodgepole is serotinous. The forest hedges its bets.
After the last ice age, however, a bridge of trees never reached the South Hills for squirrels to cross. As a result, serotiny became prevalent and the pines’ branches are laden with unopened cones, green, tan, and gray, from as far back as 20 years ago: A vast “seed bank” that steadily pays out to crossbills as the cones inevitably weather and open, flame or no flame. So crossbills are twentyfold more abundant in the South Hills than elsewhere in the Northern Rockies. The South Hills’ pine seed production is also astoundingly consistent; typically, trees have erratic crops that force squirrels to move.
Little wonder that crossbills settled in the South Hills. But when Benkman first arrived, he noticed that the lodgepole pine cones were again enormous, elephantine compared to other places, and right away he suspected that only coevolution with crossbills could explain it. Since the birds pry open the tip of a cone, rather than bite scales off from the base, the cones’ upper scales had become thicker and larger, like ramparts on a turret. Over time, the birds’ bill had followed suit, though it is less a battering ram than an exquisite, sickled crowbar.
Strolling on a South Hills ridge, Benkman and I came to a pine dripping with cones. He pinched one, tan and shining, in his long fingers. “That is a whopper,” he said. Whenever Benkman examines a monstrous pine cone, it’s as if he’s witnessing a miracle. “You could not find one this size in the Rockies. If you looked for a year, I bet you couldn’t find one.” In the lab, Benkman has shown that a crossbill’s feeding efficiency — its prying and husking time — increases on South Hills cones to a bill depth of about 10.0 millimeters, which explains type-9’s large beak. In other places, the ideal for lodgepole is 9.3 millimeters — a titanic disparity in the world of a finch. Now, wandering crossbills have trouble competing. Because of the range’s unwavering crop, the South Hills bird also breeds reliably each spring, out of sync with itinerants. They have forsaken their irruptive and irregular ways for a homesteader’s portion. No type 9 has been confirmed anywhere but the South Hills and the neighboring Albion Mountains, just 27 square miles of lodgepole forest in total.
Thus an invisible cloak of isolation has descended over the South Hills crossbill, which is a rarely observed phenomenon in birds. The classic model for speciation depends on geographic isolation. In the Galapagos, for example, the subtle Geospiza finches that inspired Darwin likely diversified when a few individuals radiated to lonely islands, founding new populations that evolved with little or no contact with their ancestors. Eventually, they diverged enough so that when these lineages came back together again, they were unlikely to produce offspring because of biological incompatibility or strong behavioral differences. But there is no geographic isolation from other crossbills in the South Hills. Only from squirrels.
As soon as Benkman arrived in the South Hills, he and his students began to survey crossbills by recording birds in the pines, including mated pairs, and diagnosing their spectrographs. From this behavioral data, Benkman and his students know that nomadic call types fly through in pulses; one year, they accounted for 20 percent of all the crossbills in the range. Yet the South Hills crossbill mates with different types — which would result in gene flow — just .7 percent of the time. That’s less than the hybridization rate among Galapagos finches, the wellspring of evolutionary theory. It’s well below the 5 percent commonly accepted as the threshold for a species. As John Thompson, an evolutionary biologist at UC Santa Cruz and a longtime colleague of Benkman’s, told me, the South Hills crossbill is a precious illustration of how coevolution could “not just generate adaptation, but also potentially cause reproductive isolation” without a geographic barrier.
Based on this evidence, in 2009 Benkman argued that the South Hills bird should be named Loxia sinesciurus, or the crossbill “without squirrels,” and, unprompted, the American Ornithologists Union (AOU) species committee considered the question: Does the type 9 crossbill merit a page in our field guides? A majority voted in favor of sinesciurus, six to five, but the bird fell one “yes” short of the seven required. One objection was a dearth of specimens, which may have spoken to Benkman’s devotion to these birds: He didn’t want to kill them. But the skins are now laid out, peach-red, in the Museum of Vertebrates at the University of Wyoming, not far from Benkman’s office. Others voted “no” because they preferred to rethink the red crossbill call types as a whole rather than piecemeal.
But in a new study in Molecular Ecology, Benkman, his former student Thomas Parchman, and several others, have at last shown that the South Hills crossbill deserves to be christened Loxia sinesciurus. They sequenced the DNA of 219 red crossbills from all of the North American call types except the rare Newfoundland mope, and ultimately compared 18,385 single-nucleotide polymorphisms, the proverbial teeth on the alphabetical zipper of a gene. When you map the overlap in these positions, you shed light on the relatedness between individuals and grow a phylogenetic tree. The red crossbill’s tree shows the bird that Benkman finally stumbled upon in the South Hills after years of exploration is a clear monophyletic group, a discrete evolutionary branch. It should end the speculation about whether the call types are cryptic species. The nomadic types may be in the early or intermediate stages of divergence, but only the South Hills bird, the AOU committee agreed, is distinct and ready for its own name: the Cassia crossbill (pronounced “Cash-uh”), after the county where it’s found.
But the origin of this species is not only the tale of lodgepole, squirrels and crown fire: It is also the story of climate, the lord of the system. And what climate created, climate can take away. Conifers appeared over 100 million years ago, but crossbills came into being much later. Why did it take so long for that ingenious bill to twist? Benkman explained that “it was pretty warm until 5 to 6 million years ago.” Cones open faster in balmy temperatures and dump their nutritious charge. “I think what was critical in their evolution,” said Benkman, “was that you had these cool conditions to hold the seeds in there.” Otherwise an overlapping bill is just funny-looking, not useful, a malocclusion instead of a machine. More recently, climate also gave us the Cassia crossbill: Models suggest hardly any lodgepole existed in southern Idaho six thousand years ago. Loxia sinesciurus must have evolved since then, making it one of the swiftest speciations known in birds.
But as Benkman and his students surveyed in the South Hills, the population crashed by 80 percent in 2011 to perhaps 1,700 birds, though it has since rebounded to about 6,000. The strongest correlation to the Cassia crossbill’s annual fortunes is clusters of days in the previous three to five years that are warmer than 90 degrees Fahrenheit — the effect on cones may mimic that of fire’s prodding tongue. To access seeds, the birds need a crevice to appear between a cone's scales as the serotiny breaks down, a miniscule gap that allows them to pry; but across the forest, they need it to happen gradually, parsimoniously, so that seeds are available year-round. Call it interest on a savings account. Benkman performed a simple binary experiment to illustrate precisely how heat waves could disrupt this economy: He gathered 900 South Hills cones and put them, in batches, in a drying oven, setting it to 104 degrees Fahrenheit and slowly raising the temperature. His patient teenage daughter monitored them through the oven’s glass window until they popped open. Then, on sunny days, he measured the surface temperatures on one tree’s cones using a thermal-imaging camera. On a 90 degree day, a cone’s plated surface might reach 138 degrees in places. At this temperature in his glorified, digital Easy-Bake, nearly all the cones relinquished. Sunlight is also a pry bar.
Crossbills in general are at risk. Since 1980, their numbers have declined about 2.5 percent a year in North America, and now they visit feeders in droves in late winter and spring, a measure of desperation. In the coldest months, the birds are scrounging — searching for a solution to the holes our industry has chewed in theirs. In the South Hills, a series of 90-degree days might result in one-fifth fewer seeds for crossbills over the next four years. Some birds might irrupt like other crossbills, but having coevolved with the South Hills pine, they aren’t suited for lodgepole as usual. “As forests go, they have nowhere to go,” Benkman told me. “Now I’m watching these birds go away. It’s just one more of the same story” — like in Newfoundland and the Cypress Hills — “only it’s not squirrels here.” But when I suggested that, in the South Hills, climate change is like an ethereal, insatiable, un-trappable squirrel, Benkman nodded and replied, “It’s a squirrel everywhere.”
Now that the bird is officially the Cassia crossbill, it is eligible for protection under the Endangered Species Act. Overnight the bird has become one of the rarest, most endangered in America. It already was. “I don’t think it will go extinct in my lifetime,” Benkman said, speculating that a band of three or four hundred birds could persist. “But who knows, if there’s a catastrophic fire, or a combination of things.” One conservation measure Benkman has imagined is to harvest unopened cones from the South Hills’ hottest, south-facing groves. You could plant those seeds in polar-opposite areas — shady north-facing slopes, cool pockets — to create islands of hardy lodgepole for future crossbill generations. More and more, species will rely on islands within islands. But the harsh fact is that lodgepole is forecast to disappear from southern Idaho by 2080. Ultimately, the name Loxia sinesciurus will be a memorial signature.
Benkman is sad, but philosophical about this prognosis. It’s morally outrageous that we'll be responsible for the demise of this species — and there are so many others like it that remain cryptic and unrecognized, especially in the lower orders: 30 percent of the world’s species could still be hidden in our midst. But by their very nature, species are ephemeral. “They wink in, and wink out,” Benkman told me. “If an ice age were to occur, as was supposed to happen before climate change, the South Hills bird would also go extinct.” In truth, a creature like the red crossbill is not one entity, but myriad populations always breaking apart and coming back together — particularly over the course of interglacial periods — even if something tips the scale, as it has, for now, in the South Hills, and a new name is warranted. This is how evolution unfolds: It’s a vast “kaleidoscope,” as Benkman and John Thompson have both called it, of intricate interactions — of crossbills, pines, squirrels, fire, climate, and more — across time and landscape, and it's always shifting. But that doesn’t mean we shouldn’t honor the Cassia crossbill and try to prolong its existence.
The best place to use your cellphone in the South Hills is from the bald summit of Pike’s Mountain, and I drove up one evening with Benkman. In all directions, stands of spiry pine and luminous aspen cling to the South Hills amid the sheep-grazed sage. We had this imbricated, somehow pastoral view to ourselves until there came a familiar whine and a trailing plume. “OK, good,” Benkman said. “There’s finally an ATV. It just wasn’t right.” They revved onto the ridge below us as a mountain bluebird plucked a caterpillar from a yellow arrowleaf balsamroot. You could imagine the Cassia crossbills flitting through these velvet swaths of lodgepole, their kingdom, and hanging from cones with bands around their legs: specks of flame-colored dust, swept up for a moment.
The famous beak of the finch: A male Cassia crossbill with its lower mandible crossed to the left. Across the population, their bills cross equally in both directions.
Photo: Nick Neely
Nick Neely is the author of a new book of natural history essays, Coast Range: A Collection from the Pacific Edge, and a former HCN intern.
This story was originally published at High Country News (hcn.org) on July 12, 2017.
To read the original story on hcn.org, click here
This story was originally published at High Country News (hcn.org) on July 12, 2017.
Photo by: Nick Neely
Photo by: Nick Neely
Photo by: Nick Neely
Photo by: Craig Benkman