This is a poor photo but it shows the wingtips of several adult Common Gulls. Note that of the three wing tips that are most clearly visible, all show a white mirror in primary 8 (primary 10 is the outermost primary). In this shot, one bird (facing with food) has a tiny triangular mirror in P8; the wingtip above this bird shows a larger more circular mirror; more interestingly, the wingtip of the bird with its back to the camera shows an extensive white mirror in P8. Note that on this latter feather, the mirror bleeds across the vane into the outer web and this extension appears as though it almost meets the white tip. Of course the dull light combined with movement of the wing may be exaggerating the shape of this mirror.