Herring Gull H 1321462cy (argenteus), April 06 2004, Westkapelle, the Netherlands (51.33N - 03.25E). Picture Pim Wolf.

A bird with a Belgian ring: Bruxelles 132146. This 2cy individual shows the common moult stage in 2cy argenteus by late-April: the juvenile wing-coverts and tertials are abraded, the primaries are bleached brown and the bill-base starts to turn paler. Most scapulars have been replaced for second generation feathers, with the last moulted scapulars still showing buffish-grey tones on the centres. By the last week of April, many 2cy Herring Gull in the Netherlands can be seen with shed P1, some P2 and even P3 dropped, though rarely.

Argenteus has a partial moult in 1cy autumn ("first pre-basic moult" or "post-juvenile moult"). There is a second partial moult of head and body-feathers in spring, the so-called "first pre-alternate moult". This partial moult starts in January and ends by May, bringing birds in so-called "first summer" plumage. The head and under-parts will turn white by June.
The juvenile tertials, lesser, lower lesser and median coverts have a notched pattern, worn by April. The juvenile greater coverts show a 'piano-key' pattern; in argenteus on the outer greater coverts as well. This pattern has gone lost as the white parts of the feathers are worn away. The juvenile tertials are very abraded as well. 
The secondaries, primaries and primary coverts are bleached, but the juvenile inner primaries still show an obvious pale window, prominent from below and above. The under-wing is rather uniform grey-brown patterned. The tail has a broad sub-terminal band and isolated dark bars on the basal half.

Research in the 1990's by H. Vercruijsse revealed that at least 50% of the Dutch argenteus from SW Netherlands are true migrants, while the other 50% stay within 100 km's of the colony (limited dispersal within the delta of large rivers in the SW of the Netherlands). Migrants follow the coastline south and the majority winter at the coast of Belgium and NW France. Once the wintering grounds are established in the first year, birds are very bound to this particular area, even wintering in the same harbour, using the same resting places and feeding grounds. This habit is found in other species as well (Glaucous Gull, Pontic Gull, etc).