Cassin’s Vireo is an interesting bird here in Colorado.  In 1997, the American Ornithologists Union determined that the former Solitary Vireo was actually three separate species and now we have Blue-headed Vireo, Plumbeous Vireo, and Cassin’s Vireo.  The default breeding species among this trio in Colorado is the Plumbeous but Cassin’s does migrate through Colorado in the spring in fall.  There are many more Colorado reports in the fall compared to the spring but definitive information on its movements is probably complicated by the identification challenges of these three species in both spring and fall.

Nonetheless, when we recently noticed multiple reports on eBird of Cassin’s Vireo in north-central Colorado , it seemed like the right time to go look.

One of our favorite nearby birding spots is Old South St. Vrain Road in the Boulder County foothills near the town of Lyons.  In mid-morning on September 6, we heard a vireo calling in the distance along the road but were unable to get a bead on it.  Forty-five minutes later, we walked past the same spot and heard the bird again.  This time, we were able to track it down and took several photos of a bird in the shadows and also recorded it singing.   After returning home, we posted photos and the recording at the American Birding Association’s popular Facebook page, What’s This Bird, and asked for feedback.  It was interesting how the conversation flowed over the next several hours and here are some of the comments:

Here are the photos I posted on Facebook along with the comment that they were cropped but otherwise not altered:

Vireo

Vireo

Vireo

And here is that Facebook conversation (edited for clarity) involving about a dozen different commenters:
  • Cassin’s Vireo, perhaps?
  • Hmm… I’m hearing shrill Plumbeous-type phrases, with none of the burry notes that I associate with Cassin’s.
  • Sounds closer to Blue-headed, but seems unlikely.
  • The notes toward the end of the recording do have some of that burry-ness.
  • Looks Blue-headed to me. Why unlikely? It is in it’s migration range isn’t it?
And now we start discussing range maps:
  • Looks like it barely, but I’m not too knowledgeable about Colorado occurrences. Now that I am looking at the maps more closely. BHVI seems more likely 
  • The range map I’m looking at shows it rare in Colorado.
  • I’m not very familiar with Plumbeous and Cassin’s though. But it gave me the Blue-headed impression.
  •  eBird reports for Colorado in September have the following percentages for frequency of checklists including each species: Cassin’s 1.7%, Blue-headed 0.1%, and Plumbeous at 2.6%. Not that these are all identified correctly, of course!
  • And the bird is what it is, probability notwithstanding.
  • This is a tangled web, though. If 10 people birding together saw a Cassin’s. That’s 10 checklists for the same bird. But if someone saw two different BHVI, it would still show Cassin’s as more likely.
And now back to the bird’s appearance:
  • This bird is way too drab for any Blue-headed. In fact, it’s even drab for a Cassin’s, but I can’t see a Plumbeous having this much yellow on the flanks. I’d call this a Cassin’s. And yes, the song does sound very Blue-headed like, but there is a lot of overlap in the burriness of “Solitary Vireo” song phrases, including eastern Cassin’s sounding like Blue-headeds and vice versa.
  •  Yellowish coloration isn’t consistent in appearance or placement on the bird, so the color isn’t accurate, but good enough to rule out Blue-headed (not that range isn’t nearly enough). Cassin’s or Plumbeous.
  • The contrast between the crown and the back is nonexistent, supporting Cassin’s over Blue-headed. Also, the cheek sort of blurs into the throat, whereas we would expect a sharp contrast for Blue-headed. Given those plumage characteristics and the differences in abundance, I’d feel comfortable going with Cassin’s.

After thanking all of the commenters on Facebook, I posted one of the photos on which I’d taken the time some post-processing (only the exposure was changed, not any of the coloration) and here it is.  This does a better job of showing the lack of contrast between the head and back color and the rather fuzzy demarcation between cheek and throat.  These weight against Blue-headed Vireo.  The yellowish cast to the underparts weighs against Plumbeous Vireo.  Interestingly, at least two other Cassin’s Vireos were reported the same date from nearby locations:

Cassin's Vireo
Cassin’s Vireo, Boulder County CO 9/6/17. Click for larger version

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