This post is not about my sightings, and may a bit dry for those not fascinated with Dark-eyed Junco population distributions and taxonomy. Be forewarned!
(I also promised more science on the blog…and a discussion of Cackling Geese. That will have to wait until I finish some more pressing science matters: my Master’s thesis! One month away…)
The common Dark-eyed Junco population that breeds in New Mexico is the Gray-headed Dark-eyed Junco, Junco hyemalis caniceps. These birds are uniformly gray above and below with dark lores, pink bill, and a reddish back.
New Mexico also has the Red-backed Dark-eyed Junco, Junco hyemalis dorsalis, which looks nearly identical to the Gray-headed Junco. It differs in having the throat and breast slightly lighter gray than the upperparts and a bicolored bill. The maxilla is dark and the mandible is dull pink. The Red-backed breeds along the Mogollon Rim in Arizona and New Mexico, but its distribution is not well known due to confusion with Gray-headed Juncos.
One Red-backed Junco have been spotted recently at the Sandia Crest House to the north and east of their usual range. The banders even captured it last weekend, allowing close inspection of plumage and size characters. Those photos don’t show the underparts or the mandible very well, but the maxilla is quite dark. I have seen Gray-headed Juncos with a dark culmen (the top ridge of the bill) and otherwise bright pink bills. Is this normal variaiton or evidence of introgression between the two populations?
Like many variable species such as Dark-eyed Juncos, the situation is even more confusing. The plumage, vocalizations, and genetics of Red-backed Juncos are closer to Yellow-eyed Juncos, Junco phaeonotus. Red-backed Juncos seem to form a bridge between Dark-eyed and Yellow-eyed Juncos. In which species do they really belong?
The more common Gray-headed Juncos at the Crest House are often misidentified as Red-backed Juncos by visitors (including myself at first!). I know the banders have caught very few Red-backed Juncos; they do occur, but very infrequently.