If you are a moderately bird-interested person who’s spent much time in Seattle or Vancouver, you’ve probably had a version of the following conversation with a less bird-interested friend or family member from out of town, after one of you spots a midsized black bird perched on the rail of the Burrard Street Bridge or scavenging in Pike Place Market:
The less-bird-interested person: “Hey, a crow!”
You: “Yeah, there’s a different species of crow around here. The Northwestern crow.”
The LBIP: “Oh, really? How are they different?”
You: “They’re, uh … slightly smaller?”
The LBIP, skeptically: “So this is one of those?”
The crow: “CRRAAAH”
You: “I … I honestly don’t know.”
That news comes from a paper that’s online ahead of print in Molecular Ecology, presenting population genomic data from American and Northwestern crows. The authors, led by David Slager at the University of Washington, collected mitochondrial DNA sequence and nuclear SNPs called using ddRADseq from crows across the northern Pacific coast, where Northwestern and American crows overlap, and some adjacent regions occupied solely by American crows. That combination of data allowed them to estimate both the extent of genetic differentiation between the two crow species — how much they are or are not reproductively isolated — and the timing of their original split.
The first major result is that, despite what your skeptical less-bird-interested friend thinks, Northwestern and American crows are detectably genetically differentiated. The nuclear SNP dataset supported two populations corresponding to the expected distributions of the two species, and the mitochondrial sequence data identified two different lineages, with an estimated time to common ancestry upwards of 440,000 years ago. Individual crows’ mitochondrial haplotypes generally aligned with their probability of assignment to one population or the other based on the nuclear data.
But that nuclear data also revealed a lot of ambiguously assigned individuals — probable hybrids, and not recent ones. Explicit analysis testing the timing of hybridization found no first-generation hybrids in the dataset. They were all a few generations down the line from mixed mating events.
The authors identify the likely timing of divergence between Northwestern and American crows as lining up with periodic glacial cycles across the northern hemisphere. Glacial expansion during cooler periods covered a lot of the Pacific Northwest with ice, but refugia remained in the coastal regions where Northwestern crows today are almost entirely un-hybridized. Northwestern crows could have split from American crows while isolated in a coastal refuge from glaciation, and begun to mingle again when the climate warmed, the ice retreated, and they could expand their range southward to come back into contact with American crows. They also note that the extent of the Northwestern-American hybrid zone is quite wide compared to similar cases across North America, suggesting that, at a genome-wide scale, there is not much selection against hybridization.
What this means for the listing in your field guide depends on ornithologists’ preferred species concept, but Slager et al. emphasize the lack of effective reproductive isolation seen in their data: "In light of our results," they conclude dryly in the Discussion, "past claims of two distinct crow species breeding assortatively in sympatry appear to have been overly ambitious".
Slager DL, KL Epperly, RR Ha, S Rohwer, C Wood, C Van Hemert, and J Klicka. 2020. Cryptic and extensive hybridization between ancient lineages of American crows. Molecular Ecology doi: 10.1111/mec.15377