Hello, Birders. Many thanks to Nick Komar for his interesting posting about a tricky vireo in Fort Collins, Larimer County, apparently on May 2nd, 2010. (Nick and all: Please always include dates. See below on why this is as important as, well, posting the name of the species observed.) Here is Nick's posting: http://tinyurl.com/23qhshj I agree with everything Nick says. Basically, this is an interesting bird, and it is confusing! Consider this image: http://tinyurl.com/23qhetl If I saw that bird in, say, Pennsylvania in early May, I would unhesitatingly call it a Blue-headed Vireo. But we're in Colorado, so Nick is appropriately cautious. Well, what is the bird? Answer: I don't know. And that brings up my main point. "Solitary Vireos," especially in Colorado, are *much more difficult* than I think most of us realize. (By the way, "Solitary Vireo" is the old name for the birds currently classified as the Blue-headed, Cassin's, and Plumbeous vireos.) I've seen birds in the fall in Colorado that are hopelessly intermediate between Cassin's and Plumbeous. But when it gets really difficult is in the spring, especially at the Cassin's/Blue-headed nexus. Here's the deal. Most of the migrant Cassin's we see in the fall are freshly molted. Relatively speaking, they're easy to identify. (Again, it's relative. I've seen plenty in the fall that are emphatically not easy.) By spring, though, many individuals are *much* duller. Many are still wearing most, or perhaps all, of their feathers from their prebasic molt, close to one year old. A spring Cassin's thus looks somewhat like a fall Plumbeous, and a spring Blue-headed thus looks somewhat like a--wait for it--fall Cassin's. There's the rub. If you're seeing birds in spring that look the nice spiffy Cassin's you saw last fall, *they're not necessarily Cassin's*. And it gets more complicated. Everything I've just said is only *partially true*. It applies to many, but not all, individuals. Many birds molt few if any feathers in the spring; some may molt a whole lot; and a few may have nothing but old feathers. Everything's on the table, ID-wise. I suspect the only unambiguous birds in the spring in Colorado are bright Blue-headeds. David Sibley, in his field guide, importantly reminds us that species limits in the "Solitary Vireo" complex are poorly resolved and that some birds simply cannot be identified. And he raises the possibility of hybrids. Now I will say that at least a few "Solitary Vireos" in Colorado in the spring are fairly "safe," from a field ID standpoint. About a week ago, Jane Stulp e-mailed me a photo that I thought was "as good as it gets" for a spring Cassin's. (I believe Mark Peterson and Brad Steger were involved in this record, too.) But most of the others are head-scratchers to me. Here's a story to ponder. A few years ago, there was a seemingly crazy report of a territorial Plumbeous Vireo in Maritime Canada in early summer. Crazy. But the bird checked out. Until it started singing just like a Blue-headed. And then somebody pointed out that the gene for color (hue, to be technical about it) in the "Solitary Vireo" complex can simply be turned off, rarely so. Just as we see the occasional leucistic robin, so a Blue-headed Vireo can be born with a gene that "turns off" its non-gray hues--making it look indistinguishable from a Plumbeous Vireo. It just goes to show that what we see isn't necessarily what's really out there. I'm aware that a lot of "Solitary Vireos" have been reported to COBirds in the past 2 weeks, but the only "sp." I've seen thus far is Nick's bird in Fort Collins. All the others, as far as I am aware, have been decreed to be one "species" or the other. Just as we start out with "Empidonax sp." for so many of our empids, so I think the default for a spring Cassin's-type has to be "Solitary Vireo sp." That's the starting point. In many instances, I don't think we'll get much past that. That's how hard this taxon is. And with empids, at least they call or even sing a lot--and those vocalizations are distinctive. But not so with "Solitary Vireos"! A few years ago at a Western Field Ornithologists meeting, Jon Dunn convincingly demonstrated to all of us that "Solitary Vireo" songs are practically useless for field ID. Unquestionably, I've had the experience in Colorado of a Cassin's-type vireo delivering a Plumbeous-type song, and vice versa. These birds are very, very hard. So let's call them "Solitary Vireos" till we know more about them. And how might we go about doing that? A great start would be to try to figure out timing of migration in the different populations (call them "species," if you will). When confronted with a report of a potentially extralimital "Solitary Vireo," the *very first thing* I consider is the date of the report. That's more important to me than "wing bars" or "spectacles" or "malar contrast" (talk about the most overrated field mark of all). In Colorado, the situation is especially complicated, since all three populations are present as migrants, apparently in both spring and fall. We just don't know yet when to expect them. But I think we can figure it out, with careful study, and then we'll be on our way, possibly, to partially mastery of field identification of the extremely difficult "Solitary Vireo" complex in Colorado. And don't forget about intermediate birds, indeterminate birds, and--heaven help us--hybrids. -------------------------------
Ted Floyd Editor, Birding Follow Birding magazine on Twitter: http://twitter.com/BirdingMagazine ------------------------------- _________________________________________________________________ The New Busy is not the old busy. Search, chat and e-mail from your inbox. http://www.windowslive.com/campaign/thenewbusy?ocid=PID28326::T:WLMTAGL:ON:WL:en-US:WM_HMP:042010_3 -- You received this message because you are subscribed to the Google Groups "Colorado Birds" group. To post to this group, send email to [email protected] For more options, visit this group at http://groups.google.com/group/cobirds?hl=en?hl=en
