The pictures above were posted on the Oregon list serv as a "mystery tern", and the following is an analysis sent to the list which provides salient points of identification that should be considered by the careful observer in Idaho. Each year, particularly in early Fall, there are now isolated reports of Arctic Tern in Idaho, and they are definitely to be looked for over larger bodies of water, either by themselves or possibly in the company of other terns.
Obviously, this is like looking through eyeglasses made for someone else, but shading of dark and light, proportions, and shape still give us some clue as to the identity of this tern. In the first picture (bird at rest), the biggest clue comes from the head and bill. Proportion to body and bill size seem to rule out Arctic Tern. Although the bird's head is slightly turned, the head slope from the base of the bill is not rounded enough for Arctic Tern, but is more like Common Tern (or even Forster's). Although difficult to clearly see, the leg length visible, especially when looking at the right leg, is too long for Arctic Tern... It does not have that "chopped off at the knees" look so evident in Arctic.
The second picture of the bird in flight again seems too large headed. Look at the leading edge of the wing in relation to the head...From this angle too much head is showing for Arctic. Also, the amount of dark on the primaries is more indicative of Common Tern.
In the original message [reporting the bird, it was stated]...that the bill was red based with a black tip. This would apparently not indicate a juvenile bird in Sept, which should have an all dark bill. My guess is that this is an adult Common Tern transitioning into basic plumage, although the lack of carpal bar could indicate Forster's...but I think that Forster's would not have as much black on the nape/hindneck as this bird has.
-- Harry Krueger
My conclusion was exactly as yours, this bird is an adult Common Tern transitioning to basic plumage.
I saw approximately 12 Common Terns off the south jetty of the Siuslaw R. (at Florence, OR) on Sunday afternoon and the head molt on these birds was nearly identical to [this]...bird. They were all adults with no carpal bar or scaly look to the back. The dark on the upper and underwing tips is entirely consistent with Common Tern and does not match the expected pattern of an Arctic. Your comments should provide some nice education for those who oversimplify ID of small Sterna.