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This week's quiz bird, with its red dish-based bill; long, narrow wings; pale coloration; and deeply-incised tail should lead us to the terns – and the medium-sized terns in the genus Sterna, at that. (Recall that the American Ornithologists' Union has recently removed from Sterna all those terns that are not medium-sized, so I guess the above is redundant.) The fact that the bird seems to have a full black cap – at least, the forehead is black, which suggests that the whole cap is – and bright-colored bill and legs suggest that it's in breeding condition and/or alternate plumage. This deduction actually makes our job easier, as we don't have to worry about subadult plumages and their very different (from those of adults) and confusing molt patterns.
The strongly bi-colored aspect of the bill should rule out Roseate Tern, but all the other real candidates – Forster's, Common, and Arctic – are still in the mix. With the picture's lighting, correctly assessing the color of the underparts (white in Forster's, pale gray in Common, and darker gray in Arctic) is problematic, so we'll have to look elsewhere, though I have to say that our quiz bird doesn't really look Arctic-like (the bill looks long, among other subtle features). But, we'll cover the bases. Wingtip pattern is another useful feature to separate this difficult trio, but, again, lighting has made this difficult to correctly assess. However, one point that we might consider is that the outer secondaries and inner primaries are quite translucent, which may provide a strike against an identification of Common Tern.
Tail pattern is useful in this trio, with Forster's being the odd-bird out, as it sports dark edges on the inside of the fork, unlike in Common and Arctic with their dark outer edges to the tail. Ogling our bird's tail, we can see that the dark edges seem to be on the outside of the longest tail feathers, so that should rule out Forster's right? Well, if we take a closer look, we should see that the bird's longest tail feathers are actually crossed near the base, such that what appears to be the bird's left-side longest rectrix is actually the longest tail feather on the right side of the tail! Thus, the dark bits on these feathers are on the inner webs, meaning that our quiz bird cannot be an Arctic Tern, nor can it be a Common Tern. The translucent remiges (primaries and secondaries) and the long and very bi-colored orange-and-black bill also support the ID suggested by our bird's rectrices.
I took this picture of an adult Forster's Tern at Villas, Cape May Co., NJ, on 22 April 2010.
The following people (listed by submission date beginning with the earliest) submitted correct answers for the December Bird Photo Quiz—Forster's Tern:
The following list shows the number of submissions for each species guessed.
The photo and answer for this quiz were supplied by Tony Leukering.