ID of Clark’s vs Western at Sterns Lake (Boulder County)

[Mark Minner-lee]

If anyone has been out to see the Western and possible Clark’s Grebes at Stern’s Lake (Boulder County) I’d love some thoughts on how you determined a Clark’s ID. Additional commentary also welcome from others who would like to lend their expertise.

The two grebes I saw both fit Western or Intermediate but not Clark’s. Flanks on both birds were dark (suggesting Western). Bill color wasn’t different. Both birds had eyes clearly with the dark supercillium area, with the suspected Clark’s having some white in the lores.

Thoughts?

Regards,

Mark Minner-lee
Erie, CO

Sent from my iPhone

[Caleb A]

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

[John Malenich]

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 

[Adam Vesely]

Mlodinow and Leukering (2018) wrote a fantastic and informative piece in Colorado Birds about identification of Western and Clark's Grebes along with thorough discussion of hybrids. Not sure if this link will work, but try this:

https://cobirds.org/Publications/ColoradoBirds/InTheScope/84.pdf

Adam Vesely

Thornton, CO 

[Mark Minner-lee]

Thanks to all for the helpful descriptions and information.

Regards,

Mark Minner-Lee

Erie, CO

Sent from my iPhone

On Apr 11, 2021, at 3:14 PM, Adam Vesely <aves...@gmail.com> wrote:

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