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.
My reading of the paper is as pretty clear evidence that they should be treated as the same species? Under what species concept are two taxa treated as separate when they cannot be reliably distinguished by phenotype, have an enormous hybrid zone, and there is little to no apparent selection against hybrids? It seems pretty clear that they are not on distinct evolutionary trajectories, even if they might have been if the populations hadn’t met again.
This seem almost exactly parallel to the case in Common Raven, where California birds were clearly separated for a long time, and diverged extensively genetically, but have since met the other populations and are merging back together (https://www.nature.com/articles/s41467-018-03294-w). I don’t think anyone has seriously suggested that California ravens should be treated as a separate species?
I am speaking as someone who has been advocating the lumping of these two sister taxa since the late 1980’s. This was largely based on pre-DNA era work of David Johnston (Biosystematics of American Crows)…
But the times are changing in term of species definitions. The rules for reproductive isolation are shifting. We have two sister taxa that are 100% sortable by nuclear-DNA. Cryptic species are an inconvenience to birders, but that doesn’t mean they are not a real and useful designation for those interested evolution and species dynamics and they seem to be popping up all over the place.
I have not read any of the taxonomic work on California Ravens, but I have read extensively on Red Crossbill evolution, Fox Sparrows, the Winter Wren complex and, of course, “Olympic Gull”. The expectation of clean evolutionary boundaries between species is being challenged everyday by emergent genome analysis technologies.
I am not qualified to make an assertion for or against keeping NOCR/AMCR taxonomies separate, but the folks who have written this paper seem to pretty clearly come in on the side of two species and they will be driving any decisions that come down from the AOU.
I understand that they were formerly distinct and separated by the glaciers, so different species. However, considering every other piece of information, are the authors arguing that they should still be considered separate species? If so, at what point would they not be?
but the folks who have written this paper seem to pretty clearly come in on the side of two species and they will be driving any decisions that come down from the AOU.
What part of the paper do you read that way? I honestly don’t see it. The closest thing I see to a statement either way is against recognising them as separate:
In light of our results, past claims of distinct crow species breeding assortatively in sympatry (Brooks 1917, 1942) appear to have been in error
But it isn’t clear how broadly that sentence is meant to be taken. Otherwise they more or less avoid the question (which is maybe smart given how arbitrary and subjective that question is).
Species distinctions require more than just genetic differentiation, at least everywhere I’ve seen discussion of the problem. They might be defined on the basis of morphology, interbreeding, whether they recognise each other as mates, whether they are on distinct evolutionary trajectories, and so on. But none of these seem to result in these being considered different.
Given the info I’m seeing the paper I have to agree with reuvenm’s conclusion. I tend toward splitting typically, but I’d interpret this as evidence to lump them as distinct populations of a single species that are mixing across a large range.
The lead author of the paper summarized their findings on twitter by saying, “Big picture: it looks like American and Northwestern crows were once evolving independently, but their lineages are now fusing back together.” I also agree with reuvenm that the authors are arguing that the two lineages should be considered members of one species.
well, they are more than an inconvenience. They basically ‘break’ field ecology unless you are wealthy enough for and have the time for genetic sequencing of every literal individual. Which is why I advocate for keeping them in some other taxonomic unit or else making formal subgenuses so we don’t lose resolution in our data or have to back everything to genus because of a difference that doesn’t matter in most cases. If you’re trying to manage a reserve, conserve crow populations, or understand the ecology of crows in general, the cryptic species thing is of very limited value and also disruptive.
(this is just my own opinion and has no bearing on whether or not iNat recognizes a given species)
You could refer all crows from this area as Corvus brachyrhynchos Group (or Complex), which would capture all the individuals that are “good” American or Northwestern species plus any hybrids/intergrades, none of which are distinguishable with binocs in the field. It would be better than Corvus sp., which could also include Common Raven.
I find it useful to reframe these species split discussions by asking the hypothetical: “If the status quo were that Northwestern and American were currently considered subspecies of C. brachyrhynchos, would this evidence strongly support elevating them both to species status?” My reading of the evidence is that it would be very difficult to argue a “yes” answer to that question.
Evidence has been piling up for lumping the two species for a while, and I suspect we will see a formal AOS proposal based on this paper in the next year.
I just refuse to ID any crows here in the Pacific Northwest of North America, where I am based. I also laugh every time I see another crow ID’d as “Northwestern”, because I neither see how they determined it was that species, nor do I see how anyone does. (Along similar lines, I also stopped offering ID’s for any gull that I expect someone might call a hybrid Glaucus-winged - Western)