Monday, May 7, 2012
Quiz #448 (2012-2-06) Solution
Solution by Tony Leukering
What was a tricky quiz to begin with was exacerbated by my poor memory. When I first cropped the original picture represented by this week's photo quiz, I had intended to put in a caveat along the lines of "Only whole birds need be identified." Of course, in the extensive time between creating the quiz photo and posting it -- at least 6 months, I forgot about wanting to use that caveat. Only upon seeing the responses did I remember such. Ah, well. So, I had to come up with a solution that I thought fair and agreed with my intent for this picture and will explain such after I provide the solution to the quiz.
We'll start with what was probably the easiest subject, the big tern in the middle with the thick-based orange bill. While we might consider Elegant, the bill is just too thick and the head pattern is incorrect, as Elegant has more extensive black on the crown behind the eye. Caspian Tern has a much-redder bill, so this is a Royal Tern. Perhaps the next easiest was actually the bird that I was trying to "hide" when I took the picture specifically for this venue, the front bird preening. It is smaller than the Royal Tern, but larger and paler than the terns in the back of the picture. It also has short, black legs and a black bill with a yellowish tip; it can be only a Sandwich Tern.
The two terns in the back on which the picture is focused are quite similar in size, upperparts color and tone, and head pattern. On the front bird of the two, we can see that the bird has some white in the forehead, unlike the back one. We can also see on both birds, though with the particulars more easily discerned on the front one, that the outer primaries are considerably darker -- essentially black -- than are the inner primaries. This, then, brings up one of the subjects on which this quiz picture is focused: molt in terns.
Though complex, understanding how and, particularly, where individual tern species molt can greatly assist in the ID process when confronted with a Sterna tern (now that only four medium-sized terns are in that genus, at least, in the ABA area). Adding to the complexity of tern molt are two other factors at odds with how we birders understand feather wear and replacement. Unlike nearly all other birds, terns have primaries that wear darker, rather than lighter. That is, they start out pale and wear toward black, whereas, wing feathers on most dark-winged birds start out dark and wear paler, often to a medium brown. This difference is due to the fact that tern primaries are black to begin with, but are covered by a pale bloom that wears off, leaving the true color of the feather. Granted, if those feathers are retained long enough, they will then bleach browner, but that doesn't happen all that often in these species. Also, these species often suspend a primary molt for migration or winter (or both!) and then resume it at the end of that interrupting period. This phenomenon can create the situation where adjacent feathers replaced in the same molt may be months different in age. This topic is too complex to deal with well in this venue, and I recommend as a primer to the subject the treatment on pp. 691-695 plus the individual species accounts in Pyle, P. 2008. Identification Guide to North American Birds, part II. Slate Creek Press, Bolinas, CA.
So, knowing that, in terns, darker primaries are generally older than are paler primaries, we can discern that various of the four Sterna terns have different molt strategies, which mesh well with their migration strategies. Considering only adults of such, and both of these two terns are adults, as discerned by the extensive black caps (and other features), we can determine that these two birds have the late-summer/fall pattern of darker outer half and lighter inner half of the primaries typical of Common Tern. Arctic and Forster's terns, as adults, do not typically show such strong inner vs. outer contrast, while Roseate typically exhibits fewer blackish primaries, thus having the break between darker and paler being farther out the wing, that is, not including so many middle primaries as in Common. Also, the one bill that we can see is too stout and long for Roseate and, especially, Arctic, and is too thin and short for that of Forster's. Had this picture been taken in spring -- the other time that we just might see dark-billed Sterna terns with entirely black caps, there would be little contrast between inner and outer primaries in adults, even of Common Terns.
That does it for the birds that I had intended the quiz to cover. However, since I omitted the intended caveat, many tackled the nearly-all-there bird in the left foreground, some better than others. This bird's upperparts coloration is more in line with that of the Royal and Sandwich terns, that is, paler than the Commons. But, the real clue to its identity is the head pattern, much of which we cannot see. However, there is enough to do the job. The nape is not black, but gray, and a mottled gray at that. This coloration seems to contrast fairly well with the suggestion of a blacker face. Throw in the dark patches on the tertials, and this is a fairly typical plumage of a second-summer Forster's Tern.
To help correct for my omission of the intended caveat and the great difficulty of correctly IDing the Forster's Tern, I have decided to not penalize those respondents that did not note the species in their answers, but to award an extra bonus point to those that did, essentially making that bird worth two bonus points. I heartily congratulate both Tyler Bell and Peter Wilkinson in arriving at that individual's correct ID. Tyler has the advantage of seeing assemblages such as this (picture taken at Cape May Point SP, Cape May Co., NJ, on 7 September 2011) regularly in Maryland and Peter is, well, he's a Brit; they are just generally better at these difficult groups than we American cousins are. Because they have to be, considering their avifauna!
One respondent's answer was precluded from being correct for the competition as it omitted most of the capitals and mis-spelled one species name. Another response had no incorrect species, but did not have enough of them.
Incorrect species provided as answers:
Black Tern - 1
Roseate Tern - 2
Congratulations to the 16 of 20 respondents getting the quiz correct:
Answer: Common Tern, Forster's Tern, Sandwich Tern, Royal Tern