That was a bit mean!
I have seen birds like this before. Back in the early 90’s in Bedfordshire. There were hardly any large gulls in the county during the breeding season. Rookery Pit held the ony breeders I saw, where I was surprised to find one normal pair of argenteus Herring Gulls plus a hybrid combination of Herring X Lesser Black-backed Gull. The latter produced 2 young.
For the second year running a family group has arrived at Slimbridge WWT in July 2011, consisting of a mummy, daddy and baby. One parent is a Lesser Black-backed Gull, the other is a Herring Gull. The youngster is still being fed by both. So it would appear to be a hybrid.
Not surprised there were a variety of answers! So no shame. Personally I would probably have gone for a juvenile Herring Gull if I didn’t have prior knowledge. Studying juvenile Herring and Lesser Back-backs was one of my key starting points for identifying the rarer gulls later. I soon found that many juveniles of both species didn’t fit easily into the field guides caricatures. There was a bit of a convergence of features. Some LBB’s with paler patterned greater coverts and slightly more patterned tertials, and certainly some young Herrings with plainer scapulars and tertials and even darker greater covert bars. Some could be real stinkers. However the seeming fail safe was the pattern of the inner primaries. Only slight variable and Herring always with obvious pale ‘window’. There is more detail involved in the exact pattern of inner primary feathers -not for now though. This bird seems to me to have tertials and greater coverts which are a bit too extreme (in terms of pale patterning) for LBB but the primary window finishes it off. It’s too Herring Gull-like. However structurally is rather slim and especially long winged and weak-billed. Bit more LBB like in this regard. So tricky but educational.
Hybrids are out there and cause problems. There is currently a bird in Shetland which is though to be the 2nd record of Yellow-legged Gull (michahellis) for the archipelago. I was asked to comment on it by a few uneasy obs. To me it’s subtle, but it reminds me of some tricky Herring X LBB hybrids. You can argue all the right ‘bits’ are there for a YL Gull but the sum of the bits looks wrong somehow. Some photos here if you want to have a look.
BTW- blame James Lees- he sent me the pics!