Watching Evolution in Action
A favorite, often unexpected, sighting for winter birders, the red crossbill provides a textbook example of how one species can split into many
David B. Williams
FEW BIRDS generate as much excitement among casual birders and professional ornithologists alike as does North America’s red crossbill. “Because the birds are nomadic, you never know when you will see one,” explains birding enthusiast Frances Wood, author of Brushed by Feathers: A Year of Birdwatching in the West. Year-round residents of cool, evergreen forests stretching from Canada and Alaska south through the Cascades, Sierra Nevada and Rocky Mountains, red crossbills depend so completely on conifer seeds for food that they routinely wander far and wide—and unpredictably—to find them, particularly during winter.
Small, stocky finches, the red crossbill (Loxia curvirostra) and closely related white-winged crossbill (Loxia leucoptera) are also the only North American birds with bills that cross at the tips like misaligned scissors. The birds hatch with uncrossed bills, which begin turning inward in about four weeks and cross completely by six weeks.
One traditional story holds that this odd bill shape arose because crossbills tried to extract the nails that held Jesus to the cross (and that Christ’s blood turned the birds red), but biologists explain it as an adaptation to the species’ strict conifer seed diet. Crossed bills enable the birds to pry apart cone scales so they can extract seeds with their spoon-tipped tongues. A tendency to hold the cones with their thick legs, hang upside down, and use their bills like legs to cling to branch tips gives feeding crossbills a parrotlike appearance.
Conifer seeds—or rather, seed shortages—are what turn crossbills into nomads. “All the cone-bearing trees the birds depend on are unreliable,” says retired Virginia Polytechnic Institute biologist and crossbill expert Curtis Adkisson. “They put out a big cone crop one year, and the next year nothing at all.” That’s when crossbills take off, turning up in unexpected places across the continent, including outside their ordinary range. Biologists call these mass movements of birds irruptions. Birders who witness crossbill irruptions call themselves lucky.
Unknown to most crossbill watchers, though, is another unusual fact of the bird’s biology: Red crossbills are to North America what finches are to the Galápagos Islands—a beautiful and clear example of evolution by natural selection. Like Darwin’s famous finches, crossbills have evolved unique bill sizes, bill shapes and body sizes depending on the kind of conifer seeds they eat. There’s a crossbill that specializes on ponderosa pine seeds, one that’s equipped to eat lodgepole pine seeds and another that feeds on western hemlock. While the birds may all look alike to a casual human observer, they seem to recognize their own kind. Each type of crossbill has a different call and refrains from breeding with other types, for example. Although the American Ornithologists’ Union officially recognizes just one red crossbill species today, biologists have proposed as many as nine.
In the South Hills of Idaho, University of Wyoming biologist Craig Benkman has been studying one type of crossbill for more than a decade. He believes the birds are in the process of evolving into a new species, with speciation driven by what he calls an “evolutionary arms race” between crossbills and the lodgepole pines the birds feed on.
Throughout most of the red crossbill’s Rocky Mountain range, says Benkman, red squirrels have the upper hand in seed retrieval, because they harvest and cache whole cones in early fall while crossbills must mine seeds from the cones that remain on trees. The squirrel’s taste for lodgepole seeds has encouraged the trees to evolve short, wide cones—more difficult for the rodents to bite off—with thick scales at the base. In Idaho’s South Hills, however, there are no squirrels. Lodgepole pines there have longer, thinner cones whose scales are thicker at the tips where seeds are housed. To pry open these cones, South Hills crossbills have developed bigger bills than other crossbills.
The absence of squirrels also means a steadier supply of conifer seeds. This has allowed South Hills birds to establish a more regular pattern of breeding than crossbills elsewhere in the Rockies, which breed irregularly and opportunistically whenever food is available. Fueling the evolution of a separate species, crossbills that fly into the South Hills from other areas rarely stay long enough to breed with the locals because they cannot extract seeds from the tougher cones.
What Benkman finds most intriguing about his research is that it illustrates two key processes in evolutionary biology—coevolution and speciation. More than a century after Darwin, biologists still have few examples that show the process of coevolution leading to divergence and new species. “I think one of the beauties of this is that people can relate to it because it involves pine cones and birds,” says Benkman. “In addition, it is occurring right in our backyards. You don’t have to go to the Tropics to see evolution.”
David B. Williams is a Seattle, Washington-based writer who specializes in conservation and natural history.
Winter Finch Wanderings
Crossbills belong to a group of birds called winter finches, all of which may turn up in unusual places during the cold months. In addition to red- and white-winged crossbills, the group includes the pine siskin, purple finch, evening grosbeak, pine grosbeak, common redpoll and hoary redpoll. When just one of these species wanders beyond its normal home range, it is known as an irruption. When several species irrupt simultaneously, biologists call the relatively rare event a superflight.