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.
February 1, 2007
Evolution & the Red Crossbill
There's a great article in the latest National Wildlife magazine comparing the North American red crossbill to Darwin's finches in the Galapagos: