A favorite way for northwest birders to pass the time on long birding road trips is to debate when, where, and how to positively identify Northwestern Crows. For most of us, one of the most challenging question from travelling birders is, “Where can I go to find a countable Northwestern Crow?” Over the years, we’ve come to realize there are almost as many answers to these questions as there are birders.
This uncertainty impacts the quality of eBird data. As individual birders, we have developed our own criteria for when and where to report sightings as Northwestern Crows, or when and where to use the ‘slash’ designation “American/Northwestern Crow”. The Northwest regional eBird editors are hopeful that genetic research currently underway at the Burke Museum will help clarify the distribution of Northwestern Crow at the southern edge of its range and the extent to which Northwestern Crows hybridize with American Crows. This information should help birders choose the right taxon (American, Northwestern, or American/Northwestern) when submitting crow sightings to eBird Northwest. For now, recent and still unpublished preliminary results of the mitochondrial DNA (mtDNA) work at the Burke Museum provides some insights on the distribution of American and Northwestern Crows in Washington and is informing eBird’s northwest editors.
Here’s the science so far: mtDNA is passed down only from the mother, so an individual crow has either pure Northwestern or pure American mtDNA, even if that individual is a hybrid or backcross. The preliminary Burke research shows no apparent Northwestern Crow mtDNA influence on the Washington coast south of Grays Harbor, on the east side of Puget Sound in the Seattle and Tacoma areas, or east of the Cascades. However, the data show that crow populations on the west side of Puget Sound and along the northern Olympic coast contain a mixture of mtDNA types; that is, some individual crows in these populations have American mtDNA and some have Northwestern mtDNA. Across the international border, crow populations in the Vancouver and Victoria areas also represent mixtures of American and Northwestern mtDNA types.
So, how does this help you since you can’t see mtDNA through your binos? Based on these preliminary genetic data and the fact that no definitive field identification criteria are known to separate these taxa, the eBird regional editors are currently implementing the following review guidelines for eBird submissions in coastal Oregon and Washington.
Active research at the Burke Museum using nuclear DNA will soon provide precise information on the hybrid status of each individual crow sampled for this study, along with a more detailed assessment of the width and extent of the putative hybrid zone between American and Northwestern Crows in our area and into British Columbia and Alaska. We eagerly await these data, but at least until then we suspect the crow-argument pastime will continue.
Article by Bill Tweit, with information and input from Dave Slager