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ID Terns, North Atlantic coast (1 Viewer)

Jan-Paul Charteris

Sussex birder and budding moth enthusiast
Look like Common to me on lack of contrast on the bill and bill structure, but that's only a feeling. I'm not too sure what would be diagnostic if anything from this angle


Well-known member
I'm not sure if it's the exposure which is lightening the bill base colour, but I suspect these are Forster's - bill structure looks a tad heavy for common. Not easy, though and I could readily be persuaded they are commons. Helpful, eh?

Jan-Paul Charteris

Sussex birder and budding moth enthusiast
If it helps I'm pretty sure someone on one of these threads has previously pointed out a difference in cap shape (either how straight it was or how it met the bill) - if it's the latter it may be possible to draw conclusions from this photo, if the former than probably not as the cap would look straighter on a flying bird than an alert 'on the deck' bird


Well-known member
Sometimes it is actually possible to learn something on BirdForum...
The question of adult breeding Forster's Tern versus Common Tern has come up several times before here, and a few months ago, Killian Mullarney posted some very helpful pointers not found in any field guide:

Based on this, the identification of the two terns in the photo becomes clearer, even though light conditions do not seem to be ideal. The left hand bird is definitely a Forster's; note extensive dark grey on central primaries, and white wedge on outers, almost like in a Black-headed Gull. Also, note bulging white loral area above bill base, and white underparts. I am not entirely sure of the right hand bird, but it looks more like a Common Tern to me: dark grey on the primaries is much more confined to the tips, there seem to be extensive grey blotches on belly and flanks, and the white loral area is much thinner.


Well-known member
This photo was taken this August. On further study I found out that the tern can't be a Forster's since in August it should being showing more white on the forehead due to molting. Very confusing isssue. There is a great essay on Birdfellow.com on this subject: http://tinyurl.com/4243har


Hmmm. That's funny
Opus Editor
United States
I believe both are Common Terns. As noted, Forster's does not have a solid black cap at this season. The tail pattern looks good for Common Tern for me. I see dark outer webs on the outer rectrices. There is some shadow on the undertail, but not the gray inner web pattern which would suggest Forster's.

The underwing pattern is similar in Common and Forster's and unlike Arctic which has a much crisper trailing edge to the primaries.


Well-known member
While Killian´s drawing shows the main points - especially the underwing feature - I don´t think it´s enough to identify Hilke´s terns. As Peter suggest for the left hand bird - the primaries from below seems to show the pattern described for Forster´s but compared to the right hand bird - this is only due to the disimilar fashion the primaries are held in, which means that the left hand bird has it´s primaries more closed. The darkish trailing edge are equal in shape in both. Further, the bill size and shape in the the left hand bird is more in line with Common and the shape of the loral pattern, the bulging over the gape line would fit some Common as well. Also it looks like the outer web on the outer t-feather is dark as noted by Joseph.


Jane Turner

Well-known member
I think that the photos are a little over exposed and lit from under the right wings which is hiding the inner wing panel of common tern.
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