This summer an entire species of crow vanished instantly. This happened not by death, but by choice. On June 30, the American Ornithological Society, which maintains a formal checklist of all North American birds, announced that it was officially absorbing the Northwestern Crow (Corvus caurinus) into the American Crow (Corvus brachyrhynchos). With this vote the organization’s Checklist Committee rendered the Northwestern Crow no more.
In our current era of the Anthropocene, the continual loss of species is something that weighs heavy on my heart, as it should—so many of these losses occur at human hands. There are instances, though, when losing a species does not come at the price of biodiversity or our own cultural identities. Sometimes it is simply the outcome of evolutionary processes that march forward without any action from us, and with no care or consideration of our opinions or life lists. This is one of those stories.
Since the mid-19th century, ornithologists have recognized American and Northwestern Crows as separate species based on a variety of features, including voice, size, behavior, and geography. The American Crow occupied the majority of Canada as well as the lower 48 states from interior Washington to Southern California, Florida, and New York, while the Northwestern Crow resided from British Columbia to Southern Alaska. As time went on, however, their differences became more obscured. While some birders were convinced that they could tell one from the other, most crow biologists like myself couldn’t discern the differences—even in the Puget Sound, where the species’ ranges met and I live. Just what, I wondered, were those crows I saw everywhere? The ones that seemed equally at home on the pier, waiting for the anglers to turn their backs on Styrofoam cups of nightcrawlers, and in the city, staring down patio diners until the patrons caved and sacrificed a French fry. Some field guides simply threw their hands up at the question, inserting question marks in the convergence zone.
My uncertainty about the identities of Seattle’s crows was a continued source of unease. After all, it’s deeply uncomfortable to consider oneself a subject-matter expert and yet lack clarity on the matter of just which subject you’re an expert on. Throughout my graduate program researching crows in Seattle, I resorted to feigning confidence. I called all my subjects American Crows. And I told the puzzled birders who pulled me aside the same: “They’re American Crows—at least as far as we know.” I wasn’t incorrect; we really didn’t have evidence to the contrary. But it felt more like handwaving than expertise. Still, I couldn’t exactly put an addendum on my publications: “This study examined American Crows. Probably. Maybe some were Northwestern Crows, or weird hybrids, who knows, lol.”
Such discomfort is, perhaps surprisingly, not all that unusual among organismal biologists. The Earth’s vast biodiversity is, frankly, confusing. Life presents and duplicates itself in such varied ways that science still lacks a single universal approach to defining a species. Among ornithologists, things are a bit simpler: Most folks recognize morphology, behavior, and genetics as three key considerations of a species’ identity. When it came to Northwestern and American Crows, their behaviors and appearance suggested there was to be good reason to suspect that the two birds were more alike than they were different. But there was still the matter of their genomic identities. So when a study examining the genetic differences between Northwestern and American Crows came out this year, I held my breath in anticipation of the findings.
Dave Slager, an evolutionary biologist at the University of Washington, and colleagues set out to determine when the two crows split from their shared ancestor into distinct species. The team examined differences in nuclear and mitochondrial DNA from specimens collected across North America, and found that speciation occurred some 440,000 years ago, when the formation of late-Pleistocene glaciers separated the crows’ intact populations. Isolated in their respective pockets of livable habitat, the formerly united species did what all organisms do in the face of new selective pressure: They changed. And from one species emerged two. Well, kind of.
Eventually the glaciers receded, removing the physical barriers that had long separated crows in western North America. Although their time apart irrevocably altered their genomes, it did not appear to make a lasting impression on their taste in sexual partners. The team found extensive hybridization between the two species. In fact, along their shared 550-mile range from coastal Washington to British Columbia, the scientists found not one “pure” individual.
It seems that while we were busy puzzling over which species was which, the crows were simply getting busy and with little attention to their partner’s ancestry. My confusion, it appears, was justified after all. Armed with this new genetic evidence, this summer the American Ornithological Society officially absorbed the Northwestern Crow into the American Crow, and the global number of extant crow species dropped by one.
To love crows is to love animals that are at once completely permeated into most of our daily lives and also on the brink of evanescence. The reason for this odd dichotomy is that there are dozens of crow species, some of which have flourished in the Anthropocene as if it were designed for them, while others, like the Åga and ʻAlalā, are barely hanging on. If the hybridization at the heart of AOS’s decision had been the result of, say, anthropogenic development that forced American and Northwestern crows together, or the impacts of human-caused climate change, I might feel more acutely the loss of Northwestern Crows.
But the study did not find this to be the case; hybridization has been going on since well before the era of colonial influence. When the AOS crossed the Northwestern Crow off its checklist, it was not the kind of heartbreaking biological disappearance that I anticipate when I think of the few wild ʻAlalā trying to evade feral cats and introduced rats in Hawaii’s degraded forests. The absorption of the Northwestern Crows is a byproduct of a complex, reticulated evolutionary history that enriches our understanding of biodiversity, rather than undermining it.
The merger of these two species represents an important advancement in our understanding of how species shift in and out of the boxes we create for them. But scientific merit aside, I also consider this move a personal favor to me. Never mind that I had zero input on the decision—I can finally assume the crow-ID confidence that I yearned for as a graduate student. Gone is the self-conscious handwaving. In its place is the kind of wild gesticulating that anticipates the joy of explaining how, among their many other superb qualities, crows can help teach us the vast complexities of evolution.
Kaeli Swift is an avian behavioral ecologist and a postdoc at the University of Washington. She received her Ph.D. from UW, where she studied the death behaviors of American crows. You can keep up with Kaeli and her research on Twitter.