How Crossbills and Other Birds Are Rewriting the Rules of Evolution

Every year when the American Ornithological Society (AOS) rolls out its latest checklist updates, birders get downright nerdy.

This is especially true when the AOS decides to split a species, adding new options to the birding menu. After all, it's the only way to pick up a lifer while watching Netflix in bed.

In 2016, for example, the Western Scrub-Jay was divided into the California Scrub-Jay and the Woodhouse’s Scrub-Jay. Birders who’d seen the Western on both sides of the Sierra Nevada got an extra tick for their North American lists.

The AOS doesn’t take these splitting decisions lightly; it can take years of vetting to weigh all the evidence and opinions. Two of the biggest points it considers are how genetically disparate the groups in question are, and whether they interbreed. (Species will sometimes mate with each other and hybridize, but the area in which they do is always small and stable.) The scrub-jays, along with every other species split in AOS history, were decided on this premise.

Biologists once thought the divergence of a species largely resulted from geographic isolation. But now, thanks in part to recent advances in DNA analysis, they're realizing that species can evolve even while living in close quarters. It’s a phenomenon known as sympatric speciation. “Clearly, geographic barriers (like oceans and mountains) are still very helpful,” says J. Albert Uy, a tropical ecologist at the University of Miami. But other factors, including preferred food, migratory behavior, and mating turn-ons, can also propel birds along “different evolutionary trajectories,” he says.

Still, not every trajectory leads to a clean split; there are debates over how big the rift needs to be for two neighboring populations of the same species to be deemed unique. The AOS may have to weigh in on this hot-button topic soon with the discovery of a potential new crossbill species. A paper published at the end of last year in Molecular Ecology argues that the South Hills Red Crossbills of Idaho should be carved out as a separate species, even though they’ve always shared their evergreen forest with other Red Crossbills. “It’s a slam dunk,” says co-author Craig Benkman, an evolutionary ecologist at the University of Wyoming. “It’s by far the most genetically distinct of all North American crossbills.”

There are at least nine other varieties of Red Crossbills across North America; each has a slightly different call and bill, designed to ravage its conifer of choice, be it Douglas fir, Western hemlock, or ponderosa pine (all of which can occur next to each other). “The genetic differences between them is related to how different their bill sizes are, not geographical distances,” Benkman, who has been studying the crooked-beaked finches for more than three decades, explains. He found that the South Hills Crossbills hardly ever interbreed with other crossbills, even though they run into each other regularly.

While all Red Crossbills eat seeds from cones, only the South Hills birds have beaks perfectly suited for feeding on their local lodgepole pines. These trees, which are in danger of being wiped out by climate change, bear uniquely shaped pinecones that differ from lodgepole pinecones elsewhere—the result of a lack of squirrels in the area, Benkman believes.

Using playback tapes, he and his colleagues discovered that crossbills were more likely to respond to calls that matched their own. This goes back to the pines: Previous studies show that calls vary based on the the bird's beak size, which is ultimately shaped by its choice of food. “When you have two-tenths of a millimeter difference in bill depth, that might not mean a lot to certain species, but in crossbills it does,” Benkman says. “They treat each other like they’re on different planets.” Because physical proximity didn't always lead to physical attraction, the two groups grew apart.

The same pattern is unfolding in the sky islands of Arizona, where researchers uncovered genetic differences among neighboring White-breasted Nuthatch populations. Much to the scientists’ surprise, two nuthatches occupying oak trees hundreds of miles apart tended to be more similar than two nuthatches living on various points of the same mountain. Lead author Joseph Manthey, who has a Ph.D. in ecology and evolutionary biology from the University of Kansas, explains that some force is driving them to become genetically different based on habitat. “It’s not speciation,” he says, “but it could very well be the beginning of it.” The Island Scrub-Jay, which lives on a single island off the coast of California, appears to be diverging in a similar way.

Sympatric speciation is being noticed on other continents as well—most notably in indigobirds, a group of African brood parasites. Rather than location, their divergence appears to be driven by the vocalizations of their hosts, which they imitate as a ploy to get free childcare. Because they only mate with birds with the same vocalizations, a separate indigobird gene pool is formed with every new host species. In addition, research shows that   Medium Ground-Finches in the Galapagos,   Band-rumped Storm-Petrels in the Azores, and the   Nesospiza buntings of the Tristan da Cunha archipelago might also be straying from their not-so-distant counterparts. Other animals, especially insects, have adopted similar routes to speciation.

The takeaway from these cases is that animals can evolve remarkably fast. The South Hills Crossbills, for instance, must have come into existence over the last 6,000 years or so. (According to the pollen-fossil record, that’s when lodgepole pines first appeared in the region, Benkman says.) The changes don't have to be exponential either. In some bird families, “there’s very, very little genetic difference between any of them,” says Geoff LeBaron, an ornithologist and Audubon’s Christmas Bird Count director. Even small tweaks in DNA might go a long way in terms of evolution.

The crossbill saga, however, is still up for discussion. Splitting the species could lead to several distinct crossbills, which is why people are looking to take it slow. "We want to get all the details figured out before we start splitting off a few here and a few there," Kenn Kaufman, birding expert and Audubon's field editor, says. ​But it's up to the AOS to make it official, perhaps as soon as this summer. If it does, it will give birders a new lesson on evolution, along with a new lifer—on the house.