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Tuesday, January 16, 2007

Chihuahuan Ravens in Boulder County

While observing a Virginia Rail and Wilson's Snipe (see post below) yesterday, I was alerted to a pair of oncoming ravens by their calls. It struck me that the calls were thin and high, unlike the Common Ravens I commonly hear around Boulder. I snapped a string of shots of the closer bird as it flew by, which provided fodder for analysis. Below the pics is my brief ID summary and discussion, posted to Cobirds this evening. Click the photos to enlarge. No adjustments have been made except for cropping and compression to make them more web-friendly. The last image has comparison images of Common and Chihuahuan Raven bills superimposed, sized and rotated to match the bird in question.

With valued input from several friends of mine, research into my sound files, and using photo comparisons of raven bills, I believe that they were indeed Chihuahuan Ravens (CHRA). A key point that Tony Leukering brought up was that the bird in the photos was an adult by plumage (lacking any brown aspect in the head), and should therefore have a fully-developed bill (younger ravens retaining the possibility of having bills not fully developed yet and therefore not reliably proportioned.) While the exact position of the nasal bristle tips is obscured by snow on the bird's bill in the photos, I believe that the extent of snow sticking far down the top of the bill suggests the long bristles of CHRA (in fact, the otherwise uniform snow ends with a little lump right about where I would expect CHRA's bristles to extend, much farther down the bill than in Common Raven [CORA].) I admit that this is speculative, though, as the actual bristles can't be seen. More importantly, and not obscured by the snow, is the overall length of the bill and the relatively steeper curve of the leading edge of the culmen (top surface of the bill), which compares nearly perfectly to a reference shot I have of a known CHRA. In the Common Raven (CORA) profile shots I compared, the bills looked distinctly longer and had a more gradual curve on the leading edge of the culmen. All of my respondents shared my regret that the tail wasn't photographed from directly underneath, but the birds didn't cooperate in this area by flying over me. However, in at least one of the shots I feel as though the view isn't inconsistent with CHRA's shorter tail with a flatter trailing edge in comparison to CORA. Finally, while I reiterate my awareness that CORAs can exhibit a wide range of vocalizations, these birds sounded just right for CHRA, further supporting the list of positive CHRA traits I observed in these birds. I've wondered if the blizzards in SE Colorado might push CHRAs out of their "core" range more than normal this winter. It has also occurred to me that the rough conditions down there might instead help them if the predictions of thousands of dead cows (with resultant scavenging opportunities) came true. Of course, we probably don't even have a handle on where the normal distribution of these birds is away from their breeding grounds, as they can be so hard to detect and document in areas frequented by CORAs and American Crows. I wouldn't be at all surprised if they regularly push into the northern Front Range. On a birding trip a couple of summers ago, Doug Faulkner commented to me that they historically ranged into Wyoming following bison herds, so places like Jefferson, Boulder and Larimer Counties shouldn't be a big stretch. Anyway, with other probable CHRA sightings at the Erie Landfill and Valmont Reservoir in the past few weeks, it bears remembering that our big black corvids deserve careful scrutiny, too- something I've usually been guilty of neglecting.

3 comments:

Veery said...

Identification can be so difficult at times. What a great idea to put the comparative pictures in the shot right along with it.

brdpics said...

As they say, a picture can be worth a thousand words!

Interestingly, we saw two Chihuahuan Ravens yesterday on a field trip to the Valmont Reservoir complex in Boulder County. Don't know if this is a good year for them around here or just a phenomenon of paying attention to black corvids.

Mike said...

That's a remarkable bit of detective work. Your photo montage of raven bills is especially impressive.

When a person is still having trouble separating ravens from crows, it's humbling to observe someone distinguish between Common and Chihuahuan Ravens. You know you're a birder when you find the challenge thrilling!