Monday, September 19, 2011
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Solution by Tyler Bell and Tony Leukering
The fact that this week's quiz bird was a tern was, perhaps, one of the reasons for the light response rate, as many/most birders are uncomfortable with tern ID. I know that it took me a long time to really work things out for the species that I see a lot, and still am very uncomfortable with the ID of a couple species that I've seen, but not really been able to study in numbers. This quiz picture allows me to discuss two aspects of ID, the one of more generality -- tern ID, the other more specific -- molt. Since Tyler Bell worked through the problem in exactly the way that I would have presented the solution (though I would have been much more wordy), we'll start with his response.
"I suppose that the two dark outer primaries are going to throw people off. If you start looking at terns with pale inner primaries and two dark outer primaries, you'll be stumped. But, it looks like the wing is in a transitional state, with P9 and P10 being irrelevant to the "normal" wing pattern depicted in field guides. The color of the bill, orange at the base and dark at the tip, and the black around the eye with the blocky shape toward the nape, are spot on for Forster's Tern."
Thanks, Tyler. Now, more words. We should notice that the two dark outer primaries are longer -- obviously so -- than the adjacent very pale primary, a feature not typical of birds. One feature that can often come into play in bird ID is actively-molting birds. Thus, knowing how many feathers are supposed to be in a particular tract -- especially the primaries -- for a given species can be most helpful in the ID process. If we count inward from the outermost primary, we should only come to nine (two dark, seven pale) before running into the chunk of grayer secondaries. Since all terns have ten primaries, we can discern that one is missing. This fact, in combo with the obvious step in primary length -- which would be partly filled if p8 weren't missing, are excellent clues that the bird is in active wing molt.
In terns, unlike the confamilial gulls, primaries get darker as they wear, rather than paler. This is because terns have a powdery bloom covering blackish primaries that wears off leaving the underlying actual dark coloration of the feathers. So, darker primaries in terns are older than are paler primaries, a very important feature of tern ID. In fact, the distinctive wedge of dark outer primaries in Common Tern is due to the fact that the inner primaries were replaced more recently than were the outer primaries; the same with the smaller wedge of darker feathers on Roseate Tern.
So, as Bryan Guarente and Tyler Bell noted, the head pattern of our quiz bird is just not found on any other ABA-area tern species. I took this picture of a molting Forster's Tern on 26 July 2011 at Cape May Point S.P., Cape May Co., NJ.
Incorrect species provided as answers:
Roseate Tern - 1
Gull-billed Tern - 2
Congratulations to the 13 of 16 getting the quiz correct:
Answer: Forster's Tern