Herring Gull HPP1cy (argenteus), December 21 1999, Westkapelle, the Netherlands (51.33N 03.25E). Picture by Pim Wolf.

The ordinary moult strategy for December argenteus, with the mantle and upper scapulars moulted to second generation feathers showing an obvious anchor pattern. There are fresh feathers in the upper row of the lower scapulars, still showing a buffish tone and a broad pale fringe. The lowest row of lower scapulars is still juvenile, as are the complete wing and tertials. The inner wing-coverts and tertials look very abraded already, with the white parts of the feathers worn away. 
The history of HPP: ringed on July 10 1999 as pullus at Orfordness and recorded the same year at Westkapelle, Netherlands on December 21 1999. By March 10 2000 it was seen at Zeebrugge, Belgium (only a few 10's of km's south of Westkapelle) and a month later it was seen on April 20 2000 at Le Portel, NW France. It was still there at May 02 & 05 2000. It moved north again in summer, with reports on July 20 2000 at Blankenberge, Belgium and on August 09 2000 at Nieuwpoort, Belgium.

The partial autumn moult in argenteus (moult from juvenile plumage into so-called "first winter" plumage) includes the body and head feathers. This moult starts as soon as the nest is abandoned (late June) and continues until January. In general, the head turns paler on throat and forehead. The breast will turn paler as well. The feathers on belly and vent will still be juvenile in most 1cy argenteus by November. From July onwards, the mantle and upper scapulars are moulted to second generation feathers, showing an anchor pattern and a dark base. The lowest row of scapulars are still juvenile by March in most 2cy birds (contra e.g. michahellis). The notched pattern of the juvenile scapulars is repeated on the juvenile lesser, lower lesser and median coverts, although the medians have paler centres. The juvenile greater coverts show a 'piano-key' pattern; on the outer greater coverts as well. The tertials have an obvious notched pale fringe and transversal bar. 
The juvenile wing-coverts, rectrices and remiges start to bleach and show wear in the fringes from September onwards. The secondaries, primaries and primary coverts are dark with a small white tip, but the juvenile inner primaries 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. The iris is dark, the bill is blackish, sometimes with a paler base, and the legs are flesh-pink.