Note the difference in primary spacing in the composite with the Danish bird and that Nelson's Gull insertet in the corner.
There are some similarities between that Nelson's Gull and the Danish bird. However, as Peter Adriaens says: ‘there are also some clear differences, not just in size and shape or bill pattern, but also in plumage.
There are subtle differences in the general colour of the plumage, pattern of tertials, shape of the lower scapulars, etc, but I think the most obvious differences are:
1) Bright pink bill, quite like Glaucous Gull (clearly duller in the Danish bird);
2) Short primary projection, again like Glaucous Gull. Note the difference in primary spacing in the composite with the Danish bird and that Nelson's Gull as an inset.
3) Primary pattern. In the Danish bird, P6-10 have dark outer webs, and P4-5 have distinct dark subterminal spots. In that Nelson's Gull, only P7-10 have dark outer webs, and only P5-6 have distinct dark subterminal spots. So I would say that this Nelson's Gull is only a superficial match for the Danish bird. When you study the details, the two are quite different”.