Lesser Black-backed Gull graellsii ET17 7cy, April 22 2002, Dintelhaven, Maasvlakte, the Netherlands (51.59N,04.02E).
An adult female ringed, left green ET17. In the top image, the difference in sex can be seen: males are stronger, more powerful and often have a vicious or mean look. In the primaries, the differences are not very clear in this example, but the white primary tips and the size of the mirror depends on gender as well. Ringed June 26 1996.