I think the photos of the WEGR and CLGR at Stern's Lake posted in Jason Cole's checklist linked below seem to illustrate what Caleb is describing here. https://ebird.org/checklist/S85225919
John Malenich Boulder, CO On Sunday, April 11, 2021 at 11:33:19 AM UTC-6 caleb...@gmail.com wrote: > Hi Mark! > > For the sake of efficiency, I'll be using WEGR to be Western Grebe and > CLGR to be Clark's Grebe. > > You would be correct that there is variation and therefore ambiguity when > it comes to the black and white pattern on the face. From my limited > experience, WEGR tends to be the species that has the most common variation > that brings facial IDs into the gray-zone, and it seems like most CLGR are > pretty straightforward. That being said, looking at a grebe from half a > mile away introduces issues, because we can't see every single bird with > close-up detail as we would like. The other primary mark that is probably > the most useful one to use year-round is the color pattern of the shoulder, > neck, and flanks. > > On WEGR, the black extends from the back of the neck to more of the base > of the neck, and that dark plumage (I say dark, because depending on the > age/molt/a bajillion random parameters, it can be black, or some shade of > gray) then goes down the shoulder close to the water. I've only been > birding for a few years, so take this with a grain of salt, but I've never > seen a WEGR in the field that did *not* have dark plumage extend down to > the shoulder that connected to the water. The dark plumage extends down the > flanks, so the bird just has a darker appearance. Where the body of the > bird meets the water on WEGR is usually dark plumage. On CLGR, there is > often much more white on the neck, and the black is restricted to the back > of the neck. The white then comes off the neck and down the shoulder, which > gives the entire front of the bird a much brighter, cleaner GISS. On adult > birds, that white/light gray extends down the flanks that are in contact > with the water. In most cases, you don't even *need* to see the head in > order to make an ID: dark shoulder always means WEGR, no need to wonder. If > where the body comes in contact with the water is pretty clearly white, > then that is most likely CLGR. > > All this being said, I should mention that WEGR x CLGR hybrids do occur > sometimes. They probably aren't super common, but I suppose it's never a > bad idea to consider this possibility in those situations when you see a > bird where all the marks are deep in the realm of ambiguity. I didn't talk > much about the bill, because although I'd imagine that's helpful on adult > birds in breeding plumage, I'm not sure how *reliable *of a mark that is. > Sure, CLGR tend to have brighter and cleaner yellow bills, and WEGR have > more dulled colored bills, but this is an even more unreliable mark to base > entire IDs off of. Just to name a few parameters that will be in constant > flux that can change the way a bill looks include lighting, feeding, and > age. > > Hope this was helpful in some way :) I'd be interested to hear what more > experienced birders do to identify the large grebes. > > *The birds are happy, and so am I* > *~Caleb Alons, Larimer County* > -- -- You received this message because you are subscribed to the Google Groups "Colorado Birds" group. To post to this group, send email to cobirds@googlegroups.com For more options, visit this group at http://groups.google.com/group/cobirds?hl=en?hl=en * All posts should be signed with the poster's full name and city. Include bird species and location in the subject line when appropriate * Join Colorado Field Ornithologists https://cobirds.org/CFO/Membership/ --- You received this message because you are subscribed to the Google Groups "Colorado Birds" group. To unsubscribe from this group and stop receiving emails from it, send an email to cobirds+unsubscr...@googlegroups.com. To view this discussion on the web visit https://groups.google.com/d/msgid/cobirds/0e340642-b4fc-4fc2-8cf6-a4c4968396f8n%40googlegroups.com.
