There are few species in North America as ambiguous as Northwestern Crow (Corvus caurinus). Even in a group of birds that are exceedingly similar the differences between American Crow (Corvus brachyrhynchos) and Northwestern Crow are minuscule at best. The only “surefire” way to tell them apart is by range; however a number of sources also cite vocal differences as a distinguishing characteristic (e.g., the Sibley Guide, some versions of the National Geographic Guide, Birds of North America).
American Crow is a very familiar bird to most people in the US – it occurs throughout the lower 48, except for certain areas near the Mexican border. Northwestern Crow is typically described as being found from northwestern-most Washington state along the Pacific coast to Alaska, as far west as Kodiak Island and the adjacent mainland. Outside of a narrow overlap zone in Washington state and southern British Colombia you can typically call the species by range alone.
But the situation is more complex than just being able to tick a bird for your list based on where you are. The American Crows found along the Pacific coast (ssp. hesperis) are at the small end of the spectrum for the species, sometimes look slightly “shinier”, and have subtly but noticeably different voices than American Crows further east (as an aside, the official designation of what constitutes hesperis is murky at best, and birds from as far away as Utah or Colorado could be this subspecies depending on some readings of the literature).
For a long time I’ve had a theory that the real “Northwestern” Crow was actually a “Pacific” Crow, since the American Crows along the northern California and Oregon coasts sounded and looked more like “Northwestern” Crows in Washington and further north, and different from the American Crows further east. If this was true, I theorized, then the crows west of the Cascades in the more humid areas of Washington, Oregon, and California, were a different species than those east of the Cascades in the drier rainshadow of the mountains.
So on a few recent trips I’ve taken to Washington I’ve made an effort to record crows wherever I found them, including going out to what were supposed to be the most “pure” Northwestern Crows left in the state, on the San Juan Islands and along the outer coast of the Olympic Peninsula. I also recorded some crows to the east of the Cascades, from the Okanogan Valley and further east. What I was hoping to find was either that the supposedly pure “Northwestern” Crows had a notably different voice than the taxonomically ambiguous ones around Seattle and along the outer coast in southern Washington, OR that all the crows west of the Cascades had similar voices that were different than crows to the east.
Unfortunately, as often happens with research of this type, what I found was different than either ideal situation. I’ve been able to hear what I feel are distinguishable differences between birds in Colorado and the ones in western Washington. But when I compared birds I recorded to the east of the Cascades in Washington I begin to hear what sounds like the start of a cline to the birds in Colorado. Recordings from further afield, in Oregon, California, and British Colombia seem to continue this cline. On the other side of the spectrum, though, when comparing the crows at the northwestern end of the range of American Crow to Northwestern Crows from Washington and further north into Alaska I don’t hear any vocal differences. This would seem to show that the end of the cline in crow vocalizations is not where the species boundaries are currently drawn.
When comparing the sounds from various crows it’s important to have an idea of what vocalizations each species makes. Anyone who has ever listened to a crow knows they have a remarkably large repertoire, everything from caws to screeches to rattles to song-like warbles. The most common calls, though, are variations on the “caw” call. These can typically be lumped into two groups, long and short, but there is a nearly complete continuum between those. Often within any individual bird the long calls sound lower and more nasal, so care has to be taken when comparing different individual crows and making assumptions about differences in pitch. Here are two cuts from the same individual bird in Washington showing a long and a short “caw”.
Ok so now to start the comparisons – listen here to what I consider a “typical” American Crow call from most of the continent, first longer calls then shorter:
And here is what I consider a typical American Crow from the Seattle area:
And a few from further east in Washington:
And here are some American Crows from further south along the Pacific coast:
The ones around Seattle to my ears sound a bit more nasal, and on average slightly lower pitched than the birds from east of Washington state. It isn’t a huge difference, and there is some overlap, but whenever I arrive in Seattle after spending time in Colorado or further east I hear the difference. The birds from Okanogan County to the east of the Cascades sound similar to the Seattle birds to my ear, but are starting to having a hint of an eastern “accent”, as are the birds from coastal California. The recording from Oregon, though, sounds more eastern to my ears, and the ones from Tulare County, California, and British Colombia sound eastern to me.
Now listen to the calls of putative “Northwestern” Crows, also from Washington:
Not very different, right? And here are links to some Northwestern Crows in the Macaulay Library from further north, far enough away from American Crow to be pure, supposedly: ML#132185, ML#136466, ML#58198.
Also not very different from the American Crows in Washington!
There are still some potential wrinkles to work out. Without knowing the sex of the crow recorded it’s hard to determine what potential differences in male and female voices there are and how that affects the variation. And it’s hard to get a real handle on the complete variation in each population without more recordings.
Now I can’t comment on the distinctiveness of Northwestern Crow based on genetics, structure or plumage, or physiological differences. But I would contend that despite what many field guides, and BNA say, there are no compelling vocal differences between that species and American Crows in the Pacific Northwest. With this lack of a vocal dividing line there seems to be little support for drawing the species boundaries in their current location. Add to that the apparent evidence of a cline between the crows in the Pacific Northwest and those east of the mountains to the birds even further east, and the evidence for having two species at all starts to look weak indeed.