Cassia Crossbills are picky eaters. The twisted-beaked birds live in southern Idaho, in two small areas of forest that cover just 27 square miles, and within those forests they eat only seeds from Rocky Mountain lodgepole pine cones. A lack of competition for food has allowed the birds to thrive there, but as wildfire engulfs one of the Cassia's two homelands, the bird's reliance on a single food source could be its downfall: Biologists estimate that as much as 40 percent of the population may be lost in coming months, a hit that could make recovery impossible for such a restricted species.
Firefighters fully contained the fire this week, and the full extent of the damage won’t be known until it's safe for biologists to survey the site. Fire maps show that 30 to 40 percent of the species’s habitat has burned, but it's unlikely that many Cassia Crossbills died in the smoke and fire, says Scott Soletti, a wildlife biologist with the Sawtooth National Forest's Minidoka Ranger District, who worked the fire and examined the forest last week. Rather, most of the affected birds likely made a mad dash to what habitat was left, spurring overcrowding that could lead to a fatal food war, says Craig Benkman, a Cassia Crossbill expert and evolutionary ecologist at the University of Wyoming.
In the near term, the birds will feast on a “buffet of seeds” freed from lodgepole cones when the blaze melted their resin coating, says Soletti. But soon, the loss of trees will mean greater competition for fewer seeds in the intact habitat. “Crossbills aren’t territorial, so it's a free for all,” Benkman says. “If you have more birds feeding on a tree [than usual], they have to share, and there’s not enough. There could be a lot of birds starving.” Before the fire, 5,800 Cassia Crossbills inhabited the South Hills, where the burn occurred, and the nearby Albion Mountains. When the final toll is tallied, 3,500 or fewer may be left, Benkman estimates.
The anticipated Cassia Crossbill losses are part of a larger die-off of birds affected by wildfires across the western United States in recent months. Burns in Washington state may have killed large numbers of sage-grouse, and fires might have played a role in the deaths of thousands of songbirds, including Grace’s Warbler, in New Mexico. The full avian impacts of this year’s record fire season are not yet known.
If the scientists’ suspicions prove out, the fire could be a death knell for this rare crossbill and its unique relationship with the islands of lodgepole they call home. Across most of their range, from the Intermountain West up through northwestern Canada, red squirrels covet Rocky Mountain lodgepole pine cones and compete with Red Crossbills—Cassia's far more common cousin. But in this small area of southern Idaho, red squirrels are absent. In their stead, Cassia Crossbills reign supreme. With an abundance of seeds, the birds never had to migrate elsewhere to find food; they stay here year-round. Through time, the local pinecones lost their defenses against red squirrels and developed thicker scales to fend off crossbills’ prying beaks. Undeterred, the birds evolved larger, tougher beaks to penetrate the cones' armor. As Benkman wrote in 2016 in The American Naturalist, in southern Idaho, “here crossbills and pine are engaged in a coevolutionary arms race.”
The Cassia Crossbill’s preference for southern Idaho’s lodgepole makes it unlikely that birds simply moved to lodgepole stands beyond its home territory to escape the fire, says Rita Dixon, state wildlife action plan coordinator for the Idaho Department of Fish and Game. “Since it’s evolved in lodgepole pine where squirrels are absent, and because of its bill morphology, it may not be as able to take advantage of other lodgepole,” she says.
“I think our alarm levels should be red,” says Zach Hutchinson, a community naturalist with Audubon Rockies. “When you have a small population that is so intricately tied to a single area, and the majority of the habitat is being lost, what will happen to that population?”
One bright spot is that the fire also burned fir trees—a useless species from a Cassia Crossbill’s point of view. Those losses could create an opening for lodgepole to expand, benefiting the bird, Benkman says.
While there are plenty of seeds in the soil to regenerate the forest, only time will tell how many new trees will survive long enough to produce more food for the birds, he says. As the climate changes, average temperatures are rising. “The question is: Is it going to be really hot, and will seedlings die?” Benkman says. And with fires becoming larger, more frequent, and more severe—all consequences of warming—the surviving trees may never get the chance to mature and produce enough seeds to propagate.
With the population under siege from so many different, compounding threats, even a small loss of Cassia Crossbills to fire can have outsize consequences.
“They have so many obstacles to overcome, and being a specialized species, it makes them especially vulnerable,” notes Lynn Snoddy, a biologist with Idaho Fish and Game, who lives about 40 miles from the fire in the town of Siler. “Any percentage above 10 percent [loss] is not good for this species.”
The Cassia Crossbill’s prospects are so dire that Benkman suspects the population may never recover. “In the short term, they dodged a bullet,” he says. "Over the longer term, over the next few decades, the habitat is going to ratchet down. They’ll lose more and more habitat. They won’t keep up with the rate of fire.”
Benkman hopes that the birds are not quite as attached to southern Idaho as he assumes. The Cassia Crossbill was only recently determined to be a separate species, distinct from the common Red Crossbill that lives across the Intermountain West, a few years ago, and there is still much about the species that remains a mystery. “That’s the thing with this species: It’s not well understood,” Hutchinson says.
It's even possible there are more Cassia Crossbills out there, in other small, squirrel-less islands of lodgepole. “Maybe there are more than we realize,” Dixon says. "But based on what we know now, this is the only place they exist.”