“Conifer scales are like shingles on a roof,” he says. “When crossbills are working on closed cones, they really have to exert a lot of force. Their bills are really over-powered for the seeds themselves. They’re engineered to get to the seeds.”
Over time, crossbill populations have come to specialize in particular seeds—such as those of black spruce or ponderosa pine—and their bills have taken on shapes that allow them to open those specific cones as readily as possible. In North America ornithologists have identified populations that specialize in at least seven different conifers; by weighing those food preferences and other characteristics, such as differing call types, they’ve proposed that there may be 10 species of red crossbills on this continent alone.
As crossbills have specialized, they’ve shaped the cones of the trees they feed on. Because getting to conifer seeds is hard work, crossbills seek out cones with thinner scales. They eat a lot of seeds, and over time their appetites have prompted conifer species to produce cones with ever-thicker scales so that some seeds are left to allow the trees to reproduce. By examining fossils, Benkman and his students have estimated how long it has taken trees to alter their cones. In Newfoundland the scales of spruce cones have grown up to 15 percent thicker in the 9,000 years since spruce, and crossbills, arrived there. On the island of Hispaniola, which has hosted crossbills for much longer, pine cone scales have become 53 percent thicker over more than half a million years.
Evidence shows this can happen even more quickly. Mauro Galetti, a conservation biologist at Universidade Estadual Paulista in São Paulo, has been studying how fragmented areas in Brazil’s Atlantic forest respond to the removal of certain animal species. In a healthy patch of Atlantic forest, such large-billed birds as toucans and the pheasant-sized guans known locally as jacutingas feed preferentially on the largest fruits of trees, like the palm trees Euterpe edulis, which locals call palmitos. Unlike crossbills, these birds don’t destroy the plants’ seeds; rather, they efficiently and widely disperse them through the forest in their droppings.
When those birds vanish due to hunting and habitat destruction, there are no animals left that can disperse the larger seeds. Galetti has shown that palmitos in areas lacking large-billed birds produce substantially smaller seeds than those in pristine areas; furthermore, they’re much less successful at having their seeds dispersed widely through adjacent areas. He has been able to show that this has happened in historical time—within the past two centuries. “The disappearance of animals from the forest is definitely affecting the evolution of this plant,” he says.
Galetti’s work is a reminder that people aren’t separate from the ongoing processes of evolution. We’ve got a heavy hand on evolution’s tiller these days. And it explains, too, why many ecologists feel such a sense of urgency at teasing out the intricacies of how the physiology of birds interacts with the environment. As Margaret Rubega notes, the feeding behavior of about three-quarters of the world’s birds has never been closely studied.
“Every time we put a high-speed camera on a bird,” she says, “we see something no one’s seen before.”