Hi Peter! I had just written up a long message with pictures, but apparently the google forum didn't like the copy-paste in photos and I lost the draft. I'll give this another try without the photos. 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 year and a half, 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. For example, the inaturalist photo you linked has an ambiguous facial pattern, but when you look at the shoulder, it's pretty clear that it's a WEGR. This particular type of ID mark is similar to Common Goldeneye versus Barrow's Goldeneye, in which you know it's Common if the shoulder is white, and it's Barrow's if that black "spur" extends down the shoulder. 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 tricky mark to make 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 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/a95161b6-1cac-47b1-b454-c6a535a336f4n%40googlegroups.com.
