Monday, September 28, 2009
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Answer by Marcel Such and Tony Leukering
Ah, another tern. I spent much of my birding life a bit scared of medium-sized terns and I still feel somewhat uncomfortable with the beasts -- particularly with non-adult Arctics and with Roseates in general. My experience with other birders in the field suggests that I'm not the only one with this problem. Medium-sized tern ID is not that easy and most of us don't really understand them all that well. This week's quiz provided another case in point, though most respondents got the quiz correct.
A few respondents provided fairly thorough analyses of the quiz that were also fairly similar. I will use Marcel Such's response to do most of my job of providing the answer. Take it away, Marcel!
"The “bridled” terns, genus Onychoprion, can be eliminated because their secondaries have a dark bar on the trailing edge that our quiz bird completely lacks. The Least Tern looks very similar, but it has pale feet in all plumages and ages. The Large-billed Tern’s name-sake bill, its bright yellow feet, and its striking pied wing pattern quickly eliminate it from the list of possibilities. The Gull-billed Tern can be eliminated because of its short, stubby bill and the species has a completely white tail, whereas on the quiz bird, R6 [rectrix 6, the outermost tail feather] is black. The Caspian Tern can be eliminated because of its massive, neon-red bill and the extensive black in the tips of the primaries. The “dark” terns, genus Chlidonias, can be eliminated because of their overall dark plumage. The big terns in genus Thalasseus, except for the Sandwich Tern, can be eliminated because of their long, colorful bills. Sandwich Tern looks very similar to our quiz bird, but like the Gull-billed Tern, it has an all-white tail. This leaves the Sterna terns as our last options.
"There are four species of Sterna terns to consider: Roseate, Common, Arctic, and Forster’s Tern. The first thing that I’ll look at is that R6 is black, which eliminates both Roseate and Forster’s terns. Separating Common and Arctic Terns can be very tricky, but it can be done by comparing several minor differences. Arctic Tern has a shorter, thinner bill than does Common; the dark tips on the primaries are much thinner on Arctic than on Common; and all of Arctic’s secondaries are white, whereas Common’s are darker, with a pale window in the inner primaries. From what I can see on the quiz bird, the bill seems too long; the dark primary tips are much too broad; and, even though we can only see the underside of the wing, the secondaries are too dark with a contrasting window in the inner primaries for the quiz bird to be an Arctic Tern."
Thanks, Marcel. I wish to emphasize the tail pattern in adult Sterna terns. Common and Arctic terns have dark outer webs to the outermost rectrices (the R6s), while Forster's Tern has dark tips to the inner webs of the inner rectrices and Roseate Terns sport entirely pale tails.
Finally, a couple respondents were, apparently, suckered by the quiz bird's pale bill tip, leading them to suggest Sandwich Tern as the bird's ID. As noted by a couple respondents, that pale tip was a red herring and the bird's tail pattern quickly eliminates Sandwich from consideration.
Incorrect species provided as answers:
Forster's Tern - 1
Sandwich Tern - 2
The 18 of 21 providing the correct answer:
Answer: Common Tern