Black Headed Gull

Wondered what a seagull was doing in a park about as far away from the sea as it is possible to get.  Checked, at home what type of gull it was and found that black headed gulls are not sea gulls at all, but very common inland. There were two of them, I waited to see if they would get close enough for me to get them both in shot, but no luck, they had obviously fallen out.