Raven I.D. question - Custer County

[Leon Bright]

COBirders—Yesterday while doing the annual Westcliffe CBC in the Wet Mountain Valley (between the Sangre de Cristo and Wet Mountain ranges at a little above 8000 ft.), , I photographed a pair of ravens perched just out of the circle. One was displaying white plumage near its wing wrist. At first sight I thought Chihuahuan Raven even though I knew that would be extremely unlikely. There was almost no wind, but while watching, I saw the second raven grooming the first, exposing apparently white plumage.

   I vaguely remember reading somewhere that Common Ravens will rarely have some white throat feathers.  I would welcome your comments as to what species of raven might be shown in the photo below. Note the raven on the right (groomer) shows no white, only iridescence. The white on the other bird (“groomee”) seems to show white plumage but not on the throat. Also notice the culmen is not as robust as that of Common Ravens.

Leon Bright, Pueblo and Custer County

 

[Nathan Pieplow]

Thanks to Leon for this excellent question and intriguing photo. When I first saw the length of the bill in the photo, my immediate thought was Common Raven. However, that exposed plumage does look pretty white.

When Andrew Spencer and I were putting together our paper on identification of ravens by nasal bristle length (https://archive.westernfieldornithologists.org/archive/V50/50(4)-p255-p259.pdf), we looked at lots of raven specimens in museum collections. The bases of many of the body feathers, not just those on the throat and neck, are bright white in Chihuahuan and silvery gray in Common. Sibley illustrates this difference pretty well. But "silvery gray" can look pretty darn white under certain field conditions.

Because Leon's photo seemed to show the left-hand bird's bill in good profile, I attempted to use digital tools to measure the ratio of nasal bristle length to bill length (see attached image). Following the methodology of our paper, I measured from the top base of the bill to the tip, instead of from the gape to the tip. The ratio I measured was 47%. My method had some built-in fuzziness, so I could be 5% off in either direction -- meaning this raven's actual ratio is probably between 42% and 52%.

As you can see from the attached figure from our paper, that would place this raven right in the middle of what's expected for Common Raven, and at the very low end for Chihuahuan. At least 90% of Chihuahuan Ravens would have a shorter bill relative to the nasal bristles than the bird in this photo.

This photo was taken in bright sunlight, as evidenced by the reflection off the right-hand bird's shoulder feathers, and I think that accounts for why the left bird's underplumage looks so white. Although I don't think it's possible to be 100% certain from this photo alone, I think these are probably both Common Ravens.

Raven identification continues to be more perilous than most people appreciate, and I am one of those who suspect that the two species may even hybridize in southeast Colorado, just to make birders' lives more difficult. Many people assume any raven east of I-25 and south of I-70 is probably a Chihuahuan, at least in summer, and I am certain that is NOT the case. Common Ravens are common breeders in the canyons of SE Colorado and they certainly wander onto the adjacent grasslands at all seasons. Card-carrying Chihuahuans are also definitely present in SE Colorado, and lots of birds are difficult or impossible to identify, especially given the distant views that are the norm in that part of the state. "Raven sp." should be the default eBird tick in SE Colorado unless good views of the nasal bristles or the feather base color convince you one way or the other.

There are also vocal differences, but variation makes them tricky to use. We need more recordings that are accompanied by definitive photos of the vocalizing bird, in order to untangle that sticky web.

Nathan Pieplow

Boulder

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