A gull with a bone to pick

At the supermarket yesterday I was taking photos of a Common Gull hoping to read its metal ring (unfortunately it was presenting the closure part) when I noticed the Herring Gull YW3T from last week.

I then noticed a Common Gull had found itself a (chicken?) bone to nibble on but it turned out I wasn't the only one to have noticed:

Where is my bone?

A pair of Greylags followed by 3 others flying over the Thames were the surprise visitors at the supermarket yesterday, as well as a pair of Coots, which I rarely see there.
Numbers of Black-headed Gulls have continued to dwindle with only 70 of them on the wharf at high tide and I am looking forward to concentrate my attention to these little guys once they're gone:

They've started being very active and vocal again. One male was calling under the eaves of one of the apartment blocks, a new site for this year as far as I am aware.