Gulls ID
Agree with Geoff that bird in photo 4 and right of photo 5 is probably not mongolicus. In addition to the remains of the fine neck streaking, which would not be expected of mongolicus as late as March, mongolicus would normally be bulkier and longer-legged with heavier bill, whereas this bird seems similar in shape/size to the heuglini "taimyrensis" in the left of photo 5 [note the thin black bill band probably indicates 3rd spring, not quite fully adult]. It would normally also have a larger mirror on p10 , extending in some to the tip, whereas this bird has an intermediate size mirror. Note also that typical mongolicus in HK have pale fleshy pink legs [but just to confuse things, birds at Lake Baikal have both yellow and pink legs including the dull peachy colour shown by this bird].
However, the distinctly paler grey mantle and yellowish-pink legs [dull yellow in one photo: dull pink in the other] indicate it is not a typical "taimyrensis" and appears to be from further east where it merges with "birulai", the western form of Vega Gull types. Typical "birulai" would have pinker legs than this, so I would say this bird is in the overlap zone. Note the current view is that both "taimyrensis" and " birulai" are hybrids between Heuglins Gull and Vega Gull, and that is why there is so much variation in the features. Thats what makes gull watching so fascinating!
Mike
