There are two individuals that have provided us with some insight: "Green F07" and "Green F02", the offpring of an American Herring and Lesser Black-backed Gull (Green F05) from Appledore Island in Maine.
This is the most well-documented hybrid available to us in North America. Interestingly, this bird has kept mostly yellow legs in 4th basic. It's easy, then, to discredit the notion that adult F1 hybrids should have a blend of dull pink and yellow legs.
This bird has been missing in action since 2nd basic. It looks much more like a 2nd cycle Lesser than a Herring (or an intermediate bird for that matter). Both size and the gray upperparts don't scream Herring Gull to my eyes.
|Green F05. Banded on Appledore Island after found hybridizing with American Herring Gull.|
This individual, known as Pierre, has been documented up and down the Atlantic, spending most of
its winters in the Daytona Beach Shores area. More here.
While in Florida a couple of weeks ago, I found two examples of adults that I feel make good candidates of this hybrid combination. Let's start with the more obvious individual.
I base my identifications of these two birds primarily on two points: intermediate upperparts and corrupted leg color. P-molt is better for a LBBG at this time of year. The head/bill on both birds look more like Herring Gull, to me.