For 20 years, Craig Benkman, a 2017 AAAS fellow, has studied the Cassia crossbill, a flocking finch species found in two lodgepole pine forests in southern Idaho, because they hold some surprising insights about the circumstances that favor the emergence of new species in evolution.
Benkman, professor and Robert B. Berry Distinguished Chair in Ecology at the University of Wyoming in Laramie, discovered the Cassia crossbill in 1996, and co-authored a paper in 2016 in Molecular Ecology that established that the bird is different enough from other North American crossbills to be declared a separate species.
Not only that, “they're a good window into a lot of processes and interactions that occur in nature,” he said.
The crossbills, whose top mandible curves across the bottom to create a prying tool for getting seeds out of conifers, are their own genus of the finch family. Ordinarily, they are highly nomadic. However, Cassia crossbills are sedentary. Only about 6,000 birds live in their two communities, and Benkman said he captures “the same birds in the same net year after year.”
Biologists have thought that animals required geographic isolation to differentiate into new species, but Benkman’s team showed that even though other crossbills fly through their territory, the Cassias don’t mate with them. A fierce preference for their flock mates apparently has stood in for geographic isolation, and has led to the Cassias’ developing a beak precisely tooled to harvest the seeds of the particular lodgepole pines that have co-evolved with it.
Benkman said all signs point to the Cassia crossbills having speciated only in the past 6,000 years or less, where most bird species have taken one to two million years to differentiate.
“This is just the blink of an eye,” he said.
Benkman grew up south of San Francisco, California, near where the Tolumne River was dammed to form the Hetch Hetchy Reservoir in the early 20th century. The celebrated naturalist John Muir, who fought against but failed to stop the Hetch Hetchy's creation, was a boyhood hero of Benkman’s.
“I wasn't a birdwatcher early on, or a botanist or anything. We just had the great opportunity to wander. It was a wonderful experience to have,” he said.
Benkman’s life and career have been informed by those youthful ramblings, but over time, he said, his focus became more scientific, his work more evolutionary. Now Charles Darwin, not John Muir, is his hero.
“Now I spend more time thinking in the office, or writing, than being outside, unfortunately,” he said.
Foraging behavior was a hot topic when Benkman was in graduate school at Northern Arizona University in Flagstaff, and later at the State University of New York in Albany, but it was difficult to measure food resources in a meaningful way.
In Arizona, Benkman became acquainted with crossbills, a far-flung group, and he was excited to realize that the birds’ method of popping open conifer cones could be studied. “You could quantify the rate at which they ate those seeds,” and estimate how many calories the birds got per unit of time. Not only that, but the conifer’s defenses were physical rather than chemical; the cones changed shape and thickness over generations to fend off the birds, setting up a “coevolutionary arms race” between them. “It seemed like a really nice system to look at.”
One problem with studying crossbills was that they often compete for seeds with red squirrels. Another was that the trees have developed some very sophisticated defenses. One strategy they use is to “mast,” producing large numbers of seeds one year and virtually none the next. Predatory birds and squirrels proliferate the first year, gorging on seeds in the year of plenty, but then they die off or scatter in the year of dearth.
“My second year of doing research on crossbills, there were none to be found,” Benkman said. Fortunately, he had some birds in his lab he could study.
Two decades ago, on his way to a meeting, Benkman found the cache of Cassia crossbills in Idaho, resident in two discrete but nearby forests in the South Hills and the Albion Mountains, a total of about 70 square kilometers. From the crossbills’ point of view, it was a kind of Eden: No red squirrels live in either area, and the trees don't mast. “They're the most stable seed producer I know of,” Benkman said of this population of pines.
The bird population is also fairly stable, or was until climate change began to heat up the forests. These particular pines are serotinous; that is, they shed their seeds in forest fires, insuring the survival of the species even in the event of a catastrophe. But even several days of temperatures above 90 degrees Fahrenheit can weaken the cones, and that is becoming more common. In those hot years, in a pulse in late summer, the loosened cones can jettison their seeds, leaving far fewer for the birds to eat throughout the year.
Climate models project 30 to 60 hot days per year by the end of the 21st century, compared with an average of one hot day per year around 1950, Benkman said. Without human intervention, if the warming trend continues, he said, “there might be a forest there, but there won't be crossbills.”