Geoff,
Thank you very much for your expert opinion and detailed explanation.
All the above 5 photos were taken at the same site, a place along the coast. The place is called Shidao ( 石島), at northeast coast of Shandong.
At this site, there were about 50 birds including Black-tailed Gulls, Vega Gulls, Yellow-legged Gulls and Mallards.
We also found these gulls in another site called Shing Shan Wai, another place along the coast. There were hundreds of gulls at this place.