Herring Gull 2cy (argentatus), May 01 2003, Boulogne/Mer, France (50.43N-01.37E).

A typical 2cy argentatus with only limited moult to second generation. 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 in birds from argenteus populations, starting with the replacement of the lower upper scapulars. This moult can also be found in argentatus from southern latitudes, as can be seen in this Danish bird and this image. The pattern of the third generation scapulars is often 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. However, 2cy birds from northern populations often show very limited moult in the scapulars, well into February and March. Examples can be found in this February bird from Murmansk, here (March) and here (April). The bird in the picture below has most scapulars still juvenile and worn, with only 25-50% of the upper scapulars moulted to second generation feathers, of which a few are very fresh. All lower scapulars and wing-coverts are still juvenile.

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 bird has P1 and P2 missing; the outer primaries P3-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.