General description:

At the landfill of Dannes, near Le Portel, mixed flocks of mainly Lesser Black-backed Gulls (1.000's) from NW and N Europe can be found including 100's of adult and sub-adult Yellow-legged Gulls. Most individuals of this page were photographed at Dannes.

As can be seen in the images, sub-adult Yellow-legged Gulls look very tidy and neat again by the end of September. Almost all feather tracts have been replaced and active moult can be found in the outer primaries. Fresh moulted wing-coverts are plain grey, with neat fringes. The fringes may be slightly paler in very fresh condition. The most striking differences between those 4cy birds in September and adults is the dark bill-band, present on both upper and lower mandible in sub-adult birds. The legs are slightly paler yellow in 4cy birds, where adults show deep saturated yellow legs.

Regarding moult pace and strategy, sub-adults don't differ much from full adults. Sometimes it's hard to age sub-adults correctly, but on average, some or all of the following features can be found in 4cy:

• relatively pale bare part coloration compared to full adults, although differences are subtle. The base of the bill is rather pale yellow, often different from the deep yellow tip of the bill. The black bill-band is often well developed in most sub-adults. The legs are yellow with a whitish wash;

• black markings on the old rectrices. Often isolated limited black spots near the centre, although most rectrices are moulted to completely white tail-feathers by September and sub-adults with old rectrices are rare;

• the grey centres of the primary coverts show a brown hue and the shaft is often an accentuated black line over the centre of the outer primary coverts;

• 4cy birds have dark centres on the greater primary coverts.

Used as a single characteristic, the evidence is not very convincing, but individuals that combine these characteristics are probably 4cy or 5cy birds. 

Moult stage and pace of remiges and rectrices:

We don't have sufficient data for detailed moult descriptions of sub-adult September birds. Regarding moult pace and timing, sub-adults much resemble full adult birds. Sub-adults can be found moulting the last central greater coverts and lesser coverts in the carpal edge in the very beginning of the month and most birds have finished the complete moult by mid-September. 
By September 21 2002, the primary moult score averages 8.1, hence most birds have P8 fully grown. 
By the end of the month the average sub-adult michahellis show active moult in the secondaries. The secondary moult is about halfway with the outer secondaries (S1 to ±S8) fully grown and the inner secondaries still missing by the end of the month.
The tail-feathers are fully grown or moult is nearly completed as can be seen in most adults as well: in those birds, the outer rectrices still have to grow an inch or so.

Complete vs partial moult in September:

The complete moult is nearly finished in September, with the last outer primaries growing and the inner secondaries still missing. In the mean time, partial moult starts again in the outer median coverts and the inner greater coverts. Upper tertials may be included in this moult as well. In the plain grey wing-covert panel, this moult is often difficult to judge, but can be seen in e.g. this bird. In the September 2cy michahellis section this partial moult in 2cy birds is explained. The extend and timing of this partial moult in sub-adults may differ slightly. 

See Topography Section for explanation of feather tracts.

Tables:

For details on differences between michahellis and cachinnans, see e.g. the article by R. Klein & D. Gruber in Limicola, April 1997.