Those of us who live in the Pacific Northwest have been arguing with the folks who put maps in field guides for decades about the status and range of Northwestern Crow (Corvus caurinus) and it is an issue of general concern for those of us wrangling crow identification for the region here on iNat. I was sent a pre-publication, not yet peer-review certified, copy of a new report by University of Washington - Slager, D.L., K.L. Epperly, R.R. Ha, S. Rohwer, C. Wood, C. Van Hemert, J. Klicka. 2018. Cryptic and extensive hybridization between ancient lineages of American crows. Pre-print server BioRxiv https://www.biorxiv.org/content/10.1101/491654v1
Here are some of the most interesting points:
Northwestern Crow is probably a good species – NOCR and AMCR diverged about 381,000 years ago, when extensive glacial periods separated populations and created conditions for genetic isolation and distinct evolutionary trajectories. Secondary contact during inter-glacial periods produced renewed gene flow and hybridization.
It appears the hybrid zone extends all the way into British Columbia (contrary to conventional wisdom).
All identified hybrids were late-generation hybrids and back-crosses, suggesting that hybrid individuals are fertile and that hybrids are the rule rather than the exception in Coastal Washington and much of Coastal British Columbia (Olympic Crow?).
There is no evidence of assortive breeding (as argued by Brooks) nor are there any consistently reliable phenotypic characters for distinguishing the two species – no reliable size differences, no reliable ecological differences and no reliable voice differences (crows learn their calls from other crows). The American/Northwestern Crow complex is genuinely and insolubly cryptic except by range or nuclear-DNA analysis.