These crow species represent distinct lineages but interbreed along the western coast of North America.
What is the difference between an American Crow (Corvus brachyrhynchos) and a Northwestern Crow (C. caurinus)? Some ornithologists argued that Northwestern Crows are clearly smaller, but that turned out to be a false impression. Perhaps they produce different sounds? That is also a tricky trait because these birds learn vocalizations from their social group. Morphology and sound are thus not sufficient to discriminate between these species. This raises the question whether they are even distinct species. A genetic approach might provide some crucial insights. A recent study in the journal Molecular Ecology explored the genomes of American and Northwestern Crows to shed some light on this ornithological mystery.
David Slager and his colleagues took a closer look at the mitochondrial and nuclear genomes of these crows. The mitochondrial analyses (based on the gene ND2) revealed two distinct lineages that diverged about 443,000 years ago. Similarly, the nuclear perspective pointed to two clusters that correspond to American and Northwestern Crows. These genetic results indicate that we are dealing with two separate lineages.
However, a closer look at the nuclear data showed that 34 individuals were actually hybrids: not first-generation hybrids (so-called F1s) but backcrosses and late-generation hybrids. These hybrid individuals could be traced back to a 900 kilometre-wide hybrid zone along the west coast of North America.
Cryptic Hybrid Zone
This study not only confirmed that there are two cryptic crow species, but also revealed the existence of a cryptic hybrid zone. Most hybrid zones have been discovered and described using morphological data. The absence of clear diagnostic traits in American and Northwestern Crows prevented ornithologists from detecting this hybrid zone earlier. This highlights the importance of genomic data in understanding speciation and hybridization. Who knows how many cryptic hybrid zones are still out there!
This finding raises a new question: what mechanisms prevent these crow species from merging into one species? The analyses did not uncover any genomic regions that were highly differentiated between the crows (so-called islands of differentiation). However, the researchers only covered a small percentage of the entire genome (they used a ddRAD approach). Whole genome analyses might be able to detect subtle differences, similar to European crows – Carrion Crow and Hooded Crow – where a large genomic region likely explains the plumage differences between these species. However, given that American and Northwestern Crow are morphologically indistinguishable other genomic regions might pop up. To be continued…
Slager, D. L., Epperly, K. L., Ha, R. R., Rohwer, S., Wood, C., Van Hemert, C., & Klicka, J. (2020). Cryptic and extensive hybridization between ancient lineages of American crows. Molecular Ecology, 29(5): 956-969.
This paper has been added to the Corvidae page.