A young Black Headed Gull. His right leg has a standard metal bird ring on. His left leg has a blue plastic one with large numbers on, designed to be visible without having to recapture the bird.
It is part of a waterbird survey to find out where the birds go after leaving their breeding areas.
A few of these have moved into the Widnes area recently. So far I have spotted 210B, 217B, 220B, 232B, and 235B.
I haven't seen this on any gulls around here but I have seen and reported a couple of Trumpeter Swans that were numbered. They had quite large numbers attached under their wings. I have a feeling it is harder to get close to them so good thing that they make the numbers large.
https://365project.org/gardencat/365/2021-04-29