Given that this conversation found me four times independently in one day, I think it's worth at least collecting some of the threads, if not measuring them or tying them together!
My perception is that the Iceland Gulls I see on Long Island and in coastal southern New England are very variable in terms of pigmentation (e.g., wingtip pattern and intensity and iris color) but not in other ways (e.g., size, structure, seasonality, habitat preference, and feeding habits). Even mantle color is close to uniform, ranging from just barely paler than American Herring Gull and Ring-bill to noticeably paler than these reference species. When I say that they are relatively uniform and distinctive in terms of size, structure, seasonality, habitat preference, and feeding habits, it's important to calibrate this within the range of options occupied by large white-headed gulls as a group, all of which are incredibly similar overall and broadly overlapping in all the ecological variables. Thus, I'm not saying that Iceland Gulls never eat garbage like Glaucous Gulls, never eat ducks like Great Black-backed Gulls, never sit in parking lots like Herring Gulls, never dabble plankton like Ring-billed Gulls, or never hunt pelagic fish like Lesser Black-backed Gulls. Most of these species do most of these things from time to time, but each has its own distinctive niche around here, and my perception is that Iceland Gull's niche is just as distinctive as that of any of the other regular species. When I say that large white-headed gulls are incredibly similar in overall size and structure, I mean to point out that the variation within species is very large relative to that among species. One way of illustrating this is to consider that basically all the possible combinations of basic size, mantle color, leg color, and eye color are occupied by at least one species, and that people have had a hard time deciding what to do when populations from different parts of the world show similar combinations: Kelp Gull was for a long time considered a subspecies of Lesser Black-backed Gull (!), Armenian Gull is perversely similar to California Gull, etc. Perhaps the most extreme and vexing example of this phenomenon involves American Herring Gull and European Herring Gull, which, although basically identical as adults, are apparently not closely related. In view of all this, our Iceland Gulls seem very distinctive and stable to me, in every way but one (or two): wingtip pigmentation (and maybe iris pigmentation). The reason that I don't call these birds "Kumlien's Gulls" is that to do so would be to imply that we regularly see or expect to see any non-kumlieni Iceland Gulls. I do not believe that nominate Iceland Gulls occur here regularly (an odd vagrant from time to time is possible but would not be identifiable), nor do I think that, among our Iceland Gulls, those with paler wingtips are more closely related to nominate birds, or that those with darker wingtips owe this to Thayer's or Herring or other ancestry. As described above, it is always possible that we could be tricked from time to time by some completely different species or hybrid combination that happens to line up with a similar character combination, but this should sort out under the weight of long-term evidence. Because we have a series of carefully written books, we know not only that this variability in wingtip pigmentation has been around for more than a hundred years, but also that the pigmentation distribution has changed rather markedly over that time. Writing in 1923, Griscom clearly implies that the Iceland Gulls that he perceived as regular on Long Island had white wingtips. This is because he explicitly cites one specimen and one sight record of Kumlien's Gull, proving that observers were aware of and looking for the possibility of wingtip pigmentation. By 1964, Bull described a very complex situation which defied simple summary, but in which it is clear that Iceland Gull was perceived to have increased in overall frequency, and that both "white-winged" and "gray-winged" individuals were well represented. Nowadays, it is clear to me that the white-winged end of the distribution is much scarcer than it was previously (although it is still encountered fairly regularly). Thus, wingtip pigmentation in North American Iceland Gulls is and has been variable, and it has shifted toward the darker end over a century of observation. And why shouldn't it have done so?--characters evolve all the time. The notion that this shift has been a consequence of hybridization between white-winged, glaucoides-like birds and Thayer's Gulls, although widely accepted among birders, seems like special pleading to me, and much less plausible than the simple alternative of character evolution within a variable population. This is because the frequency of a trait of hybrid origin is not expected to exceed the rate of hybridization, relative to the overall population sizes. Thus, the hybridization hypothesis would require not only that hybridzation occurs regularly, but that it occurs at extraordinarily high rates relative to the overall population sizes. Needless to say, detailed, quantitative evidence of this kind simply does not exist. Also, if the hybridization hypothesis were true, why have our Iceland Gulls remained so consistent and distinctive in size, structure, and ecology—indeed, in every respect other than pigmentation—rather than evincing Thayer’s influence in these other respects? It is true that we also lack direct evidence that natural selection has been pushing pigmentation along, but this seems much more parsimonious to me, based on current evidence. Shai Mitra Bay Shore ________________________________ The Campaign for CSI: For College and Community<http://www.csi.cuny.edu/foundation/> -- NYSbirds-L List Info: http://www.NortheastBirding.com/NYSbirdsWELCOME http://www.NortheastBirding.com/NYSbirdsRULES http://www.NortheastBirding.com/NYSbirdsSubscribeConfigurationLeave.htm ARCHIVES: 1) http://www.mail-archive.com/nysbirds-l@cornell.edu/maillist.html 2) http://www.surfbirds.com/birdingmail/Group/NYSBirds-L 3) http://birdingonthe.net/mailinglists/NYSB.html Please submit your observations to eBird: http://ebird.org/content/ebird/ --
