Dear Steven and Tweeters,
I don't want to step on toes here, but I hope that you are certain about the ID of the Pacific Loons. I have been out there at Rosario quite a few times when there were the usual large numbers of Red-throated Loons. Then I come home, look at eBird, and see that there were other birders there at the same time that I was there--but those birders had called the loons Pacific Loons, and did not even have the Red-throated Loon on their eBird checklist. This has happened as recently as a few weeks ago.
About 25 years ago, a friend of mine bought himself a brand-new video recorder, a big and clunky one that was state-of-the-art at the time. He and I had discussed a set of CBC data that had included large numbers of Pacific Loons at Rosario Head, and hardly any Red-throated Loons. Even back then, we were both of the opinion that the large winter flocks of loons at Rosario Head were almost always Red-throated Loons, not Pacifics. We went out to Rosario Head with the new-fangled video camera and "filmed" the large flock of loons. Then we went back to his house and viewed the video on his television set. Just as we had concluded while viewing the flock through our scopes, all of the loons that we could see on the tape were Red-throated Loons. 
On the infrequent occasions that I see large flocks of Pacific Loons in Skagit County, they are most often at Washington Park, March Point, or Samish Island.
Checking my own personal records for the two species, I find that I have seen "large" flocks (> 5) of Pacific Loons only 8 times in Skagit County over the past 30+ years. Only one of those occurrences was at Rosario Head, 8 birds on 11-03-2018. By contrast, I have seen flocks of over 5 Red-throated Loons at least 85 times in Skagit. The great majority of those were at Rosario Head.
One thing that I look for on Pacific Loons is the shape of the head. I like to call them "Puffy-Heads." They often seem to have extra-thick feathers on the crown, making their head seem plush or puffy. There is often also a subtly distinct color to the head, a slaty grey that looks different to me than the color on the head of the RTLO. This can be seen sometimes even when the birds are so distant that it becomes difficult to judge the shape of the bill.
I have not birded all that much at Semiahmoo, but I have seen large flocks of Pacific Loons twice in fourteen visits there, which makes me think that Semiahmoo is a far better place to observe large numbers of Pacific Loons than anywhere in Skagit.
All of this of course is a necessary prelude to any discussion of the ID of an Arctic Loon in Washington.
Yours truly
Gary Bletsch
On Saturday, February 5, 2022, 11:24:01 AM PST, Steven Dammer <dammerecologist1990 at gmail.com> wrote:

Hey Tweets, 
Am currently scoping at Rosario Head and there are a good amount of Pacific Loons just to the right of the island. I did encounter one that stood out. 
I'm not foolish enough to fully believe that what I saw was an Arctic Loon among the crowd, but when seen side by side with so many other PALO I couldn't help but notice how much white was visible along the flanks showing above the water line. I observed it til it dove and have not been able to relocate yet. 
Are any of y'all adept at Loon ID and can possibly help me out here? 
Best,
Steven Dammer _______________________________________________
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