This hybrid is said to be increasing in Eastern North America. Particularly interesting is the increasing amount of them that have been seen by birders in east-central Florida. It would seem that adults are much easier to sort through, and not many 1st cycle birds or intermediate ages are reported - for obvious reasons.
Here's one I recently found that I feel fits this hybrid mix:
|Presumed LBBG x HERG or Appledore Gull. Lake County, IL. 21 Dec 2013.|
Size-wise the bird was no bigger or smaller than most of the surrounding Herrings.
Again, note the pattern of the head-streaking, leg color and bill pattern.
The wingtip pattern is unlike a typical LBBG, but this is usually the case with these presumed Appledore Gulls. If these LBBGs are pairing up with so-called "Northern" American Herrings, then the big white tip to P10, medium-sized mirror on P9, and white tongue tips on P5-P7 make perfect sense.