Monday, August 23, 2010
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Answer by Tony Leukering
The quiz bird flew by me while I was in a boat in Perdido Bay, AL (at Orange Beach), 9 August 2010, while I was working on post-spill efforts -- specifically, monitoring sea-grass beds. I had been photographing Sandwich Terns and grabbed one picture of the bird as it flew by me, never putting a binocular on the bird. When I got the pictures home to NJ, I noticed the bird's bill -- no yellow. Hmm. Then I noticed the depth of the bill. Hmm. That's odd, it almost makes it look like a small gull. Hmm.
Medium-sized tern ID is difficult, and made more difficult for many by the fact that there are very few places where one can see a lot of all of the species. In the interior, one is usually left with only Common and Forster's as regular options. In coastal New England, one adds Arctic and Roseate, but Forster's drops out. In New Jersey, Forster's and Common both breed in large numbers, with Gull-billed in small numbers, but Arctic and Roseate are rare. In the West, Common and Forster's are the only game in town, really. In the far north, Arctic and Common hold a near monopoly except in parts of coastal Alaska, where Aleutian is an option. The Gulf coast sports Gull-billed, Forster's, and Common (at appropriate seasons), but few (if any) of the others. South Florida may have the best medium-sized tern diversity, with four regular species.
All of these species sport black bills at one or more seasons and ageing the bird might help us winnow the possibilities. Typical of first-cycle terns, our bird has something of a secondary bar (the left wing's secondaries are dark-centered; we cannot see the top side of the right wing secondaries, so cannot accurately assess their color) and the tail seems to have dark corners. With an age as a probable juvenile given, the facts that our bird seems to have palish legs and a pale bit in the middle of the bill might be explained (juvenile soft-part colors are often different from those of adults and can change rapidly from that when in the nest). Our bird's apparent face mask sets it apart from Aleutian, Arctic, and Common, all of which sport some black on the crown. The extent of black onto the nape should enable us to eliminate Forster's from consideration at this age -- it can't be molting into or out of a full black crown at that age. Roseate and Arctic don't show so much dark on the underside of the wingtips and also sport rather dainty bills. In fact, we don't really need to look at much else other than the bill, because, by itself, it identifies this Gull-billed Tern.
Incorrect species provided as answers:
Common Tern - 4
Forster's Tern - 2
Congratulations to the 9 of 15 getting the quiz correct:
Answer: Gull-billed Tern