Posted by Phil Pickering (208.46.229.41) on April 19, 2001 at 22:49:08:
In Reply to: Marin Gull posted by Steven Mlodinow on April 19, 2001 at 09:14:49:
>>> The intrusion of black proximally on the lower mandible is not exactly typical of Glaucous, but a review of Grant's gull guide show this feature on a number of European Glaucous Gulls (and therefore without GW Gull genes, presumably). <<<
I would agree that it's possible (but I think debatable) that a pure Glaucous could have a small amount of bleeding or bending of the black on the tip in along the cutting edge of the lower mandible. I've seen this on a few Glaucous that seemed pure in all other respects. In fact, MOST Glaucous have the black extend inward noticeably farther on the lower mandible than the upper. However, on the pure-looking Glaucous that I've seen that show bleeding, and with the photos in Grant (some of which in my opinion might be of birds showing introgression with Herring anyway) the black extends at most back about 1/3 of the total bill length from the tip. In my (humble) opinion, with the Marin gull it's a matter of extent that can be easily measured. The duskiness along the cutting edge of the lower mandible of this gull extends in much closer to 2/3 of the total bill length from the tip. I can not recall ever seeing a Glaucous-like gull, either in person or in several hundred photos I've viewed, with this great an inward extent of the black - that did not also show atypical structural and/or plumage features (typically trending toward Glaucous-winged or Herring). My strong impression is that this constitutes a pattern indicating that this much bleeding is due to introgression. By extension, perhaps even a small amount of bleeding of the black along the cutting edge would be a first detectable sign of introgression (although that speculation might belong in the "most birders could care less" column).
Cheers,
Phil