Black Headed Gull

I spent 1/2 hour wandering around the park, shot off 50 or so frames with high speed continuous and auto focus and got nothing in focus: not even backgrounds as I was panning with the birds.
Towards the end, the birds were at least in frame and impressively large, but still out of focus.
So, back to single shot to catch some lichen and then this one pops into view!