It IS a Ring-billed Gull

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     Since Mark Hoffman's analysis is "uneven", and full of "fallacies", I
must write in his defense.   
      I saw the gull at Conowingo on a weekday in early January. Based on the
descriptions others had given me of the tail pattern, I believed that I was
looking at the correct individual. It was an obvious Ring-billed Gull. I had
been in England a week earlier, where I had observed dozens of first winter
canus (=Common Gull) at close range in London parks. The Conowingo bird did
not really look like any typical canus. It would be a very extreme canus. The
head and bill shape were typical of Ring-billed. The bill color was more
typical of Ring-billed than of canus. The shade of gray on the mantle,
paleness of the greater covert panel, and the contrast of both with the median
coverts and primaries were all typical of Ring-billed Gull. While these
feature are variable, and overlap to a degree,  together they strongly
suggested Ring-billed. I was too far to observe median and greater covert
markings, but other features pointed so obviously to Ring-billed that I was
convinced that the bird was not canus.

      Susequently, I saw Mark Hoffman's very good photos, and Bob Ringler's
video. They confirmed that I had seen the same bird. Most importantly, they
gave detailed looks at the pattern of the greater and median coverts. Mark has
written a careful and detailed analysis, which together with his photos,
argues persuasively that the bird is a Ring-billed Gull.
  
   The only feature on this bird that approached canus was the very nice, neat
tail band and very clean white tail above the band. If you have looked at
Ring-bills you know that a few birds in a hundred will approach this pattern,
with an almost canus-type tail. A few Ring-bills in a thousand will have a
tail and covert pattern seemingly identical to that of a typical canus. Gulls
are just terribly variable. There is no way one can identify all gulls
correctly using a single feature. The Peterson system does not work here. One
must try to evaluate a large number of features together, and,  (with gulls
especially!) some of these features will contradict each other. Even so some
birds will not be identifiable. In this case the evidence of all other
features points to Ring-billed Gull. The Tove article in Birding is excellent,
but it did not have room to go into all the possible variations in these
species. The tail band (at least for Ring-billed), is far more variable than
is shown in the illustration in that article. Despite the tail pattern, I
would not even call the Conowingo bird a strange Ring-billed Gull. It is a
Ring-bill with an uncommon tail pattern. 
      Given that both species have crossed the Atlantic, it is not impossible
that they might hybridize from time to time. But in this case, since all
features (including the tail band), match the range of variation in Ring-
billed Gull, I see no particular reason to think that this bird was a hybrid.

      

Dave Czaplak
Germantown MD

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