Herring Gull 5.412.502(argenteus), October 2003 - February 2004, IJmuiden, the Netherlands (52.27, 04.33E). 

Ringed as fledged 1cy in the Netherlands: ringed Vogeltrekstation Arnhem Holland 5.412.502. It was ringed on September 09 2003 at IJmuiden (52.28N 04.35E) as weakened 1cy. It was held in captivity for more than 24 hours before it was released again at IJmuiden harbour. As is standard for 1cy argenteus, all the wing-coverts are still juvenile. All lower scapulars are still juvenile. One upper scapular has been moulted to second generation, two others are missing.
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.

image below: 5.412.502 1cy, October 23 2003, IJmuiden. 

image below: 5.412.502 1cy, November 11 2003, IJmuiden. 

2 images below: 5.412.502 2cy, February 12 2004, IJmuiden. 

Between late October and February, the moult in the scapulars has become obvious. The second generation greyish feathers on the flank and in the neck also strongly contrast with the old bleached brown feathers on the belly. In the image below, in frontal view, the pattern of the post-juvenile moult in the under-parts is even more clear.

images below: 5.412.502 2cy, February 28 2004, IJmuiden.