This looks like it could be a “romantic” moment between the loons, but when I checked about loon behaviour, I found out that these loons are actually rivals, and are doing the circle dance.
They slowly circle around each other, checking each other out. I saw them doing this quite a bit.
Often one would dive down, and the other would put its head underwater. This is to check to see what the other is doing. At that point it might dive below after it.
I saw them diving below, putting their heads underwater, and even doing it at the same time.
Apparently it’s rare that they actually fight. But they do sometimes. I didn’t see any actually physical contact between them.
