I live in Victoria, BC, on the southern tip of Vancouver Island. This is within the purported range of northwestern crows (Corvus brachyrhynchos caurinus). It was once considered a species in its own right but was demoted to a subspecies of American crow (C. brachyrhychos) on June 20, 2020.
Because there are no reliable morphological ways to distinguish the subspecies, it means that they are generally not identified on iNaturalist because, while species-level identification is always correct, a subspecies identification is usually suspect.
Still, as I say, Vancouver Island falls entirely within the purported range of this former species. As seen here (note this page still shows it as a species)
https://www.birdatlas.bc.ca/accounts/speciesaccount.jsp?sp=NOCR&lang=en
Then there is the issue of possible hybridization, which muddies the waters as well.
I am curious as to whether a survey of DNA from feathers collected in an area would be able to determine what percentage of crows in said area are C. brachyrhychos versus C. brachyrhychos caurinus versus a hybrid of the two.
Personally, I’d be curious whether the percentage varied much between the crows down by the water and those farther inland by even a few hundred metres (to my eye, the former are slightly smaller on average).
It would be interesting to find out, for instance, that one crow in 50 away from the water is C. brachyrhychos caurinus versus one crow in 20 down by the water.
Of course, I don’t even know if this would be possible via collected feathers.
Thanks for these, Stewart!
Though the science is a little (okay, actually quite a bit) over my head, I find it interesting that (if I am reading things correctly) the hybrid mix on the southern tip of Vancouver Island leans more to C. brachyrhychos caurinus than C. brachyrhychos. I wonder if, as suggested to me elsewhere, there are really no non-hybridized northwestern crows in the area.
Prior to the lump, if I remember correctly eBird reviewers curated eBird observations such that all crows in BC were considered Northwestern and all crows in Washington were considered American. I think everyone knew they were hybridizing but without any means of distinguishing either hybrids or pure birds that was the compromise that was decided on. It made the range maps look rather strange having a straight line at the country border!
The hypothesis is that the ancestral crows got split up by the ice sheets, with Northwestern Crow being isolated in the areas of Alaska not covered by glaciers. When the glaciers retreated the two populations slowly expanded back towards each other and they hadn’t been isolated for long enough to be reproductively incompatible. Since Vancouver Island is right at the southern extent of the ice sheet I would’ve expected American Crow to expand there earlier, but perhaps there are some differences in their ecological niches that made Northwestern Crow genetics more competitive along the coast.
You definitely can get DNA out of feathers, but in practice it is tricky. Many samples often fail, the DNA is contaminated by bacteria etc, and the DNA tends to be lower quality. Nevertheless, you can certainly often get more than enough for phylogeography.
Victoria seems to be a pretty homogenous mixture of the two lineages with slightly more caurinus ancestry than non-caurinus ancestry according to estimates in Slager et al. 2020. When two lineages meet and blend like that to create such a broad gradient, you don’t tend to get heterogeneity at fine scales - you don’t tend to see full caurinus individuals living next to full American individuals, because the two “parental” lineages (if any pure parentals still exist) are separated by a vast range filled with hybrids, and crows don’t disperse far enough for non-hybrids (if any still exist) to encounter each other. Rather, at any given location, it is more expected to see a relatively homogenous population with some % ancestry from the two species, with this % being about the same for all individuals within a given population but slowly varying across the landscape as you go from north to south.
Since it is a pretty continuous gradient, it is not particularly useful to identify them to subspecies, since to do that you would need to decide where to draw the line - how much caurinus ancestry does a member of the hybrid swarm need to have in order to count as caurinus?
