Herring Gull 42840312cy (argentatus), May 02 2003, Boulogne/Mer, France (50.43N-01.37E).

A 2cy argentatus with a Danish ring: Copenhagen 4284031. Ringed as pullus  on June 24 2002 in Katholm, Nordborg (54.58N 09.50E). 
From late May onwards, argentatus has a complete moult during summer to so-called "second winter" plumage. This complete moult will be finished by October. The head appears white from May to July and will be moulted by July-August and will show neat fine streaking by October. The under-parts and rump turns pale, mottled with brown. From mid-April, the scapulars and mantle are replaced by third generation feathers, starting with the replacement of the lower upper scapulars. As can be seen in the image, the pattern of the third generation scapulars is more or less similar to the anchor pattern of the second generation feathers, but they can be recognized by the warm buffish tone and neat white fringe as long as the feathers are fresh. In this bird, most scapulars are worn second generation feathers with 1-25% of the upper scapulars replaced for third generation feathers, still second generation-like in their pattern.

From late April, the inner primaries are shed in argentatus and new second generation primaries grown in. The last juvenile primary (P10) will be shed by mid-August. The new second generation primaries are dark with a tiny pale crescent on the fresh flight-feathers. Again, the four inner primaries show an extensive pale window, as both inner and outer-web are pale greyish white-brown, contrasting with the outer-wing. A pale inner-web can still be found in P7. This Danish bird has P1 to P3 missing; the outer primaries P4-P10 are still juvenile.
The complete tail is still juvenile in this individual. The second generation tail-feathers will show a clear-cut blackish tail-band with only isolated blackish markings on the white basal half. Most tertials are still juvenile and very worn by May, except tertial #4, which has been replaced for second generation. All the wing-coverts are still juvenile.

The characteristics of Herring Gulls from Denmark are somewhat intermediate between argenteus and argentatus.