This afternoon I had a little more time and was treated to squadrons of Herring Gulls dropping in to bathe in the nearest pool - not something they do very often and I think it is possibly related to wind direction. In amongst these I managed to pick out a couple of individuals showing the thayeri pattern on P9 as well as a couple of darker individuals with much white in the primaries - presumably northern argentatus Herring Gulls. In addition there were plenty of Common Gulls and Black-headed Gulls, as well as single Great Black-backed and Lesser Black-backed Gulls.
Biggest puzzler was the following bird which resembled a hybrid Herring Gull x Lesser Black-backed Gull but on reflection must be a subadult northern argentatus Herring Gull. One difficulty with its identification is that these northern Herring Gulls normally have a restricted amount of black in the primaries but this bird has black markings extending to P5. Of course this may be normal for birds of this age. It may, of course, be a darker than normal southern argentatus.
Obviously it is the upper bird in the first photo, which shows a pale mantled argenteus Herring Gull in front. The second picture shows the spread wingtip pattern.
The underwing can be seen in the shot below and this is followed by another shot of the bird on the water bathing with a Herring Gull.
Finally, some shots of the bird perched with argenteus Herring Gulls. Notice that the contrast between them and the argentatus appears less. In the last shot the difference between the two gulls does seem to be appropriate for northern argentatus and local argenteus.
Thanks to feedback from Stephen W and Andy (tittletattler) on BirdForum on this bird.
Here are some other pictures taken at Alnwickhill today:
Lesser Black-backed Gull (note the Black-headed Gull advancing into breeding plumage in the foreground - not the full hood it looks like from this angle but half-way there)
thayeri-pattern Herring Gulls - two different individuals both with thayeri pattern on P9 (not quite visible in upperwing shot)
Goosander (left of centre) and preening Great Black-backed Gull (far right, honest!)
Grey Heron, Mute Swans and Canada Geese
Not a bad haul for Alnwickhill!