Herring Gull 1L5 2cy (argenteus), April - August 2003, Boulogne-sur-Mer, France (50.42N,01.34E). 

A ringed argenteus from the island of Guernsey in the Channel. Tertials and all wing-coverts are still old juvenile. The upper scapulars have 5 fresh third generation feathers and one feather missing. All lower scapulars are second generation, except two rear scapulars in the lowest row, hidden by the scpaulars above in this sitting bird.

Argenteus show a partial moult of head and body-feathers in spring. 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.
By April, some argenteus will show three generation of scapulars: still a few juvenile scapulars in the rear lowest row (first generation), a few very fresh lower upper scapulars with a clear buffish tone and a neat white fringe, still with a similar pattern as the second generation feathers (third generation) and most scapulars are slightly worn upper scapulars and upper lower scapulars (second generation). 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.

top image: 1L5 2cy, April 30 2003, Boulogne-sur-Mer, France (50.42N,01.34E).

bottom image: 1L5 2cy, June 27 2003, Boulogne-sur-Mer, France (50.42N,01.34E).  

This bird clearly started the complete moult (basic moult): the new second generation P5 is just viisble underneath the longest tertial. P8-P10 are still juvenile. All secondaries are still juvenile. Moult in the rectrices has been started as well: the central tail-feathers R1 & R2 are missing.
The upper two tertials have been replaced, #3 is growing and #4-#6 are still juvenile.
Greater coverts: #1-#2 are fresh, #3-#4 are growing, #5-#17 are missing, the outer greater coverts are growing in descendant order, hence, the moult wave is initiated in the outer greaters and new feathers grow in from outer-wing inwards, meeting the wave which was started in the inner greaters at about greater covert #8.
Median coverts: #1-#13 are fresh,
Lower lesser coverts: #1-3 are fresh, #4-#5 are growing, #6-#10 are fresh fully grown, #11-#13 are missing. #13 is the last visible feather in the folded wing.
Lesser coverts: 40% of the lessers is visible, including 30% fresh second generation feathers, 40% missing feathers and 30% juvenile abraded feathers.
Almost all upper scapulars (75-100% category) are third generation, with a second-generation like pattern. A minority of the lower scapulars (25-50% category) has been replaced as well. 

bottom image: 1L5 2cy, August 08 2003, Boulogne-sur-Mer, France (50.42N,01.34E).  

This bird almost finished the complete moult (basic moult): P8 is fully grown. In the wing-coverts and tertials: tertials tt1-tt5 are fully grown, Greater coverts #1-#4 are fully grown, #5 is growing, #6-#8 are fully grown, #9 is growing, #10-12 are fully grown and the greater coverts further outwards are fully grown as well probably. The median, lower lesser and almost all lesser coverts are fully grown as well.