Comparing Common & Pacific Loons

We finally got some seasonal weather around Colorado's Front Range, with snow last Tuesday night into Wednesday morning and cold temps since then. On Wednesday, my sidekick Garrett & I saddled up and headed out to have a look at a couple of loons that had been lingering around Longmont, a Common and a Pacific.
The first order of birding in snowy, cold weather is to bundle up- think layers. Since it is hard for me to take a pic of myself, here's the G-man in his winter birding outfit, ready to roll:

Finding both birds was pretty straightforward- but both were being lazy, sleeping & preening near the middle of their respective lakes, putting them well away from any shoreline and associated predators. Plus the light was flat, light snow flurries came and went, and it was a bit breezy. Not great photographic conditions, but quite realistic for late fall loon watching. So I decided to video both birds and put together a little comparison video.

If the loons were together one of the most obvious features to compare would be the size differences, but they weren't even on the same lake, and other good size reference species like Western Grebes weren't present, either. But even with distant lone loons, much can be inferred from structure and field marks that are visible at long range.

I suspect that the Common Loon is a juvenile, but it could be a basic adult. Even at 200 meters or so, you can see the somewhat flat-topped & blocky head, white above the eye, fairly large & slightly upswept bill (underside curved upward a bit more than the topside is curved downward.) Also note the whitish partial collar intruding into the dark nape, creating an uneven border between the white throat and the nape. In contrast, the Pacific Loon (a juvenile) has a rounded head profile, curving smoothly back to the nape. The bill is relatively smaller and more symmetrical, and the nape line is straight, contrasting sharply with the white neck. Somehow the even curve of the head and neck combined with the clean dark nape reminds me of a cobra with its hood puffed up. When the bird turns its head away, the silvery-gray coloration on the nape is more visible. The forecrown of the bird and the foremost part of the nape are darker gray, almost black.

To get the shots I videoscoped the birds with my Leica 77mm APO-Televid scope with a 32X wide angle eyepice, connecting a Leica C-Lux 2 camera with their Digital Adapter 2 (plus my homemade cable release bracket.) Anyway, check out the video I put together to compare the two: