SOUTH HILLS — There’s a bird species that lives exclusively in the Magic Valley.
That should sound like an outrageous statement. Why would a genetically distinct animal that can fly — that isn’t physically trapped here — tether itself to such a tiny geographical area? What about the Magic Valley could be so biologically unique that it gives rise to an entirely new species of bird?
In two small mountain ranges in the south-central-most part of south-central Idaho, a special set of conditions over the last 5,000 or so years have led to the Cassia crossbill, a fist-sized bird with a bizarre beak that feeds on the weather-beaten cones of lodgepole pine. About 85% to 90% of all Cassia crossbills live in the South Hills, with the remaining birds living in the Albion Mountains just to the east.
If you know where and how to look, Cassia crossbills are relatively easy to find in the South Hills, turning up wherever there are mature lodgepole stands. When you come across a flock of the red males, olive-green females and brown juvenile birds they’ll often be cheeping boisterously from the treetops, using their big, crossed beaks to jack open pine cones and snag the seeds with their extra-long tongues.
The Cassia crossbill eats only lodgepole pine seeds, and the 90,000-acre Badger Fire killed at least 20% — the U.S. Forest Service doesn’t know the exact number yet — of the South Hills lodgepole the bird needs. The massive fire, which burned through nearly a third of the South Hills, might have pushed Idaho’s only endemic bird toward an earlier extinction.
Some Cassia crossbills could be in the blackened burn area right now because the heat from the fire probably opened up a lot of cones, letting the birds get at a bonanza of seeds. But once those seeds fall to the ground, where the crossbills can’t eat them, some birds will starve. Too many will have to compete for the same scarcer resource.
“Over the next couple months, their population is going to plummet,” said Craig Benkman, a University of Wyoming biologist who has studied the Cassia crossbill since the late 1990s.
Sarah Harris got her first good look at a Cassia crossbill while taking a cross-country skiing break with her family in the warming hut at the Diamondfield Jack campground.
“There actually was a crossbill that had flown in there and was kind of beating itself against the window trying to get out,” the local Prairie Falcon Audubon president said. “I took my coat off and I threw it over the bird, and grabbed it, then I had a really good chance to look at it. I got to see its crossed bill up close and personal.”
That memory, of holding an olive-green crossbill in her hands and sharing the experience with her kids and husband, is one of several reasons Harris feels a special connection to the bird. But the Cassia crossbill is charismatic enough to fascinate even non-birdwatchers. On the surface, the finch is an enigma.
Generally speaking, crossbills are nomadic, flocking to areas with good cone crops, moving around from year to year. The Cassia crossbill is a homebody, staying in the South Hills year-round.
Its bill is a biological marvel. The top and bottom of its beak don’t align, a bit like how your thumb and pointer finger overlap when you play the got-your-nose game with a kid.
That big, crossed beak is a massive advantage for a bird that spends its life prying open pine cones. The Cassia crossbill wedges its specially adapted beak into a crack in a cone, then when it bites down the tips of its beak move farther apart, widening the gap. With its crowbar beak propping open the scale, the crossbill gobbles up the exposed seed.
At first, it seems a bit odd that it’s a separate species, too. The bird doesn’t look any different from the red crossbill — the two were considered part of the same species until three years ago. To the untrained ear it’s difficult to tell them apart by sound. And to the naked eye, the Cassia crossbill’s beak doesn’t look much different. Compared to the type 2 red crossbill subspecies, its average beak size is 0.25 millimeters bigger.
“If I didn’t know much about these birds and hadn’t been studying them for the last 40 years … I would think two-tenths of a millimeter doesn’t mean much,” Benkman said. “And it probably doesn’t mean a huge amount, but it means enough that they’re more efficient, on average.”
So what’s so special about a small bird that lives on lodgepole pine seeds in a small area of forest in south-central Idaho, that’s ostensibly just a red crossbill with a bill that’s evolved specifically for South Hills and Albion Mountain lodgepole pine cones?
It has an incredible origin story.
“It’s a textbook example of coevolution,” Benkman said.
A land free of puffy-tailed competition
The Cassia crossbill exists because one mammal’s conspicuous absence has dramatically altered the South Hills and Albion Mountains.
The mountain ranges have no tree squirrels.
About 5,000 years ago, the planet entered a relatively cooler period and the forest in the South Hills and Albions expanded. But the forest didn’t grow in the desert around the mountain ranges. Squirrels couldn’t make it to the sky island without a forest bridge.
Red squirrels feast on pine seeds. They’re such good seed predators that trees have adapted their survival plans accordingly so that at least some of their seeds escape the squirrels.
When squirrels are present, conifers will often produce cones that open up relatively quickly, dropping seeds to the ground. They’ll also typically release a lot of seeds simultaneously, as opposed to consistently releasing the same number of seeds every year.
Seeds sitting in a cone are convenient, pre-packaged meals for squirrels. If the tree drops its seeds sporadically and scatters them about, the odds of those seeds surviving and growing into a new tree are far higher, so long as they sprout fairly quickly once they hit the ground.
Other strategies are more efficient if there aren’t any squirrels around and fires are frequent.
Ninety percent of South Hills and Albion Mountains lodgepole have serotinous cones. Serotinous cones stay glued shut until a fire burns through, melts the glue, and releases the seeds.
“Those cones tend to stay in the tree for a very long time; they can remain for decades,” said Nathaniel Behl, a biology teacher at Metropolitan State University of Denver and a former graduate student of Benkman’s who spent five summers working with Cassia crossbills.
There are a lot of advantages to holding onto your seeds and releasing them after a fire. Growing conditions are optimal: There’s less shade, less plant competition in the understory and fewer predators.
With a stockpile of cones in the treetops, and no squirrels to compete with, nomadic crossbills had an opportunity to settle down.
“Because there are no squirrels in the South Hills you have high seed production, consistent seed production and the seeds becoming available to the birds at a consistent amount not just year-to-year, but also throughout the year,” Behl explained.
The arms race
The absence of squirrels provided the Cassia crossbill — the bird’s Latin name, Loxia senesciurius means “crosswise squirrel-less” — with an opening. That opening explains why South Hills and Albion Mountains lodgepole exhibit unusually high levels of serotiny and why there are so many crossbills here.
In the last 5,000 years — the blink of an eye in evolutionary terms — the crossbill and the lodgepole have been influencing each other in ways that clearly illustrate several important evolutionary processes.
“(The Cassia crossbill) has evolved faster than probably any other bird species in North America,” Benkman said, “perhaps as rapidly as any bird species in the world.”
A crossbill, like pretty much any animal, wants the easiest meal it can get. Cones with thinner scales are easier meals. Thin scales are weaker, so it’s easier for the crossbill to insert its crowbar. The crossbill’s preference has helped the thick-coned trees.
“Trees that have cones with thin scales don’t have as many seedlings after a fire (because of the Cassia crossbill),” Benkman explained.
The average fire interval between lodgepole stands in the South Hills and Albions is about 120 years, Benkman estimates. So every 120 years, over the last 5,000 years, each progressive lodgepole generation has had slightly thicker-scaled cones.
The crossbills with bigger bills were better at opening those cones and had more reproductive success.
“The bills got larger and larger,” Benkman said, “(as) the cones got tougher and tougher.”
The resident crossbills kept growing bigger bills, allowing them to outcompete any nomadic red crossbills that showed up. Crossbills mostly breed within their flocks, so the residents didn’t have many offspring with the nomads. Benkman said Cassia crossbills only interbreed with red crossbills about 1% of the time.
“They became completely reproductively isolated,” Benkman said. “This allowed genetic differences to increase.”
Turning up the heat
The Cassia crossbill doesn’t just need lodgepole pine.
“(Cassia crossbills) are really reliant on old lodgepole pine,” Behl said. “Cone production is a result of tree age. The older trees are, the more cones they produce.”
As the climate changes and big fires become more common throughout the West, old lodgepoles could become increasingly scarce. More fire and hotter conditions mean fewer trees overall, and they can also mean fewer old trees.
The Cassia crossbill’s population is directly correlated with the health and size of the South Hills and Albion Mountains lodgepole forests, which are spread across just 27 square miles. Between the Badger Fire and the Cave Canyon Fire eight years ago, the Cassia crossbill has lost 25% of its range in the South Hills during the last decade.
“Let’s say we lost 50% in the South Hills (in the Badger Fire),” Benkman said as a hypothetical. “Well, if we lose 50% basically of those gray cones — the gray, closed cones — we’ve lost 50% of the crossbills by the end of the winter.”
There weren’t many Cassia crossbills to begin with before the Badger Fire. A few years ago, Behl worked to estimate the Cassia crossbill population and came up with roughly 5,800 birds, give or take a couple thousand. That’s a lot of crossbills per acre of lodgepole — about 20 times the crossbill density of a lodgepole forest with squirrels — but it’s still a small number for an entire species. Having such a small population means the bird can’t afford to lose too much habitat. Before the Badger Fire, the South Hills only had 4,000 acres of lodgepole.
More frequent and bigger fires put that tiny amount of critical habitat in jeopardy.
Lodgepole evolved with fire and needs fire, so Cassia crossbills need fire, too. But from a Cassia crossbill perspective, there’s a sweet spot for fire intervals and size, and changing climate conditions and a century of fire suppression might be pushing the South Hills and Albions out of that sweet spot.
Benkman explained that theoretically, if you wanted to manage the forest exclusively for the benefit of Cassia crossbills, you’d burn small percentages, frequently. If the fire interval for a given stand is 120 years, you could burn one-tenth of the forest every 12 years. That way you’d always have 10 different age classes, ensuring that crossbills have a perpetually consistent supply of old, weathered cones.
Climate change and fire suppression are making fires bigger, hotter and more frequent. Suppression has left more fuel in the forest. Rising temperatures have increased droughts, drying out vegetation and fuel. Climate change has led to more variable annual precipitation and an earlier melting snowpack. If fires come through too often, there won’t be as many old stands.
“All of these are not good for the pine,” Benkman said. “These younger stands, they’re 20 years old, they’ll carry fire now when you have drought.”
Lodgepole pine come back from fire relatively quickly — that’s the advantage of serotiny. They can start producing cones when they’re 10 years old. But the young trees, in the 10- to 20-year-old range, don’t produce serotinous cones until they’re about 30 years old.
“That’s going to be a real problem,” Benkman said. “If you have two fires, 20 years apart, there’s no recruitment. … Until they’re 30 to 50 years old, all the cones open in the end of the summer, early fall. They don’t produce serotinous cones. They don’t stay closed and hold seeds.”
So far, the South Hills hasn’t been losing many young, green stands in fires, U.S. Forest Service District Wildlife Biologist Scott Soletti said.
“It’s not something that’s super commonly observed in the lodgepole pine eco-type,” Soletti said. “That being said, things change.”
He noted that where the Badger Fire ran into the eight-year-old Cave Canyon Fire burn scar, “it basically went out.” Those lodgepole mostly survived.
Still as bad as fires seem today, climate change is expected to make them even worse as the West gets warmer and drier.
“Projections are that fires are going to be more frequent, more intense, more severe and larger,” Benkman said. “It’s kind of scary, because another 20% (of the South Hills) could burn next year, or in the next five years. I don’t think that’s unreasonable.”
The Badger Fire isn’t the first big event this century to threaten the Cassia crossbill’s existence.
“Around 10 years ago I was starting to think I was going to watch them go extinct in my lifetime,” Benkman said. “And now I was hoping they weren’t.”
Heatwaves hit the South Hills in 2003, 2006 and 2007. It was so hot during those summers that the lodgepole pine’s serotinous cones opened. The heat effectively melted the glue on the cones the way a fire would.
The seeds fell to the ground, creating a significant food shortage for the Cassia crossbill. The population dropped 80%. After four years without heatwaves the birds bounced back, but not to where they had been before.
This time, with so much habitat lost, Benkman said he doesn’t expect the Cassia crossbill to bounce back as much.
The Cassia crossbill’s extinction has always been at the back of Benkman’s mind. He’s known for a long time that the bird probably won’t be around much longer.
Back in 2010, Nicholas Coops of the University of British Columbia and Richard Waring of Oregon State University published a paper that looked at how climate change could affect lodgepole pine distribution. Based on their models, by 2080 lodgepole will occupy just 17% of its current range, retreating from the South Hills and Albions and virtually all of Idaho, Washington and Oregon.
Soletti said that lodgepole declines on that scale aren’t a guarantee. It’s difficult to predict future climate conditions.
“Those feedback loops, the further out you project them, the less certainty you have,” he said.
But Coops and Waring’s paper is an educated guess about what the lodgepole forest will look like in the not-too-distant future. The best guess doesn’t bode well for Cassia crossbills.
“The Cassia crossbill was, over much longer time scales, always sort of going to be an evolutionary dead-end,” Behl said. “It was always going to be a short-lived, temporary species.”
Yes, the Cassia crossbill wasn’t going to be around for millions of years, Behl said. There are still reasons to worry about its decline though.
“In many ways, the reason why we should care is because they’re emblematic of a much larger, much bigger problem,” he said.
Human-caused climate change is threatening thousands of species around the world. The best way to help the Cassia crossbill, and prevent a devastating loss of biodiversity, is to curb CO2 emissions and reduce climate change, Benkman said. Both he and Harris — the avid birdwatcher — noted that, so far, people appear unwilling to do that.
When the Cassia crossbill goes, it won’t go with a bang unless one big fire wipes them out. It probably won’t become a celebrity like the polar bear, shown starving, ribs showing, walking pitifully over shrinking sheets of ice in international documentaries. There won’t be a Cassia crossbill version of Lonesome George, the Galapagos tortoise, one last bird serving as a mascot for the species.
Instead, the Cassia crossbill could fade away almost invisibly. It might increasingly interbreed with red crossbills and lose its specially adapted genes. Its extinction could be a slow, unseen absorption back into the species that it began to leave 5,000 years ago.
There will be hundreds, thousands of Cassia crossbills across the world. Species will keep winking out, more and more every year.
“It’s sad and it’s scary,” Harris said. “For me, it will make my life a little bit less wonderful.
“I guess I’m glad I won’t be around if and when there aren’t any more Cassia crossbills.”
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