General description:

Early July and from July 26-30 2001 we did small surveys on Yellow-legged Gulls at Le Portel (near Boulogne-sur-Mer) and Etaples (near Le Tourquet). In these surveys, we scored moult stages to determine timing of moult in primaries (and wing- coverts and body-feathers) in sub-adult michahellis.
By June, sub-adult michahellis occurs only in small numbers at Le Portel and the near surrounding. But by July, more Yellow-legged Gulls disperse to northern quarters to rich feeding grounds and to moult the primaries. At the dumping site of Dannes, near Le Portel, mixed flocks of mainly Lesser Black-backed Gulls (100's) from NW and N Europe can be found including tens of adult and sub-adult Yellow-legged Gulls.

Regarding identification: the difference between 3cy birds and 4cy birds can best be established by the outer primary, which is still unshed by July: 4cy birds show a small but obvious white mirror on P10, while 3cy birds have the (second generation) outer primary all brown. In birds with very worn outer primary tips it may be difficult to ascertain whether a mirror has been present or not. Other clues can be found in the bare part coloration, with 4cy strongly resembling adults.

Complete moult in July:

As can be seen in most of the images, sub-adult Yellow-legged Gulls look very untidy and mottled in their July plumage. Simultaneous moult is in progress in the primaries, wing-coverts, tertials, scapulars and the tail-feathers. Fresh moulted wing-coverts can be found in the median row and outer lower lesser coverts in most cases and these fresh grey median coverts normally strongly contrast with the brown-hued lesser coverts and old greater coverts. The old coverts have very abraded fringes and often a brown hue. By early July, the upper tertial has been shed in many birds, as is the case with the central tail-feathers.

Moult stage and pace of remiges and rectrices:

The table below shows the average score of immature Yellow-legged Gulls, range between 3cy birds to 5cy birds. Individuals in 3cy show no mirror on P10, 4cy birds have various wing-coverts, tertials or tail-feathers with black markings and 5cy birds are very adult-like. All birds showing black on the bill combined with dark outer primary coverts were included in this score. It's hard to predict the score of new primaries; too many resting and sleeping birds could not be scored on the new fresh grown inner primaries (over 35% of the immature birds present were not scored in detail and are clustered in the table under "P?"). The average score of old primaries is 8.1, hence on average less than two old primaries are still present in the last week of July.

In 3cy birds, the early-July score averages 7.0 (n=33 on July 10 2002), so three old primaries left. By the end of July, the average primary moult score is 8.5 (n=39 on July 29 2001; n=53 on July 30 2001), only slightly higher than 4cy michahellis.

In adult birds, the early-July score averages 6.3 (n=66 on July 11 2001), so more than three old primaries left in most birds. By the end of July, the average primary moult score is 7.6 (n=50 on July 28 2001) and 7.7 (n=56 on July 29 2001) in adults, so slightly behind compared to 4cy michahellis. Adult birds were selected on plain grey primary coverts.

By the time P4 is fully grown, 2cy birds normally start the complete moult in secondaries and rectrices. 3cy birds start slightly later, when P5 is fully grown, although individual variation may exist. Sub-adult michahellis probably follow a moult timing similar to 3cy birds. Old tail-feathers may still show much black, typically found on third generation rectrices, but old feathers are not necessarily third generation in michahellis. As can be seen in previous sections, tail-feathers may be included in the partial autumn moult in both 1cy and 2cy, so old tail-feathers in July sub-adults may be completely white as well. However, the new white tail-feathers can be recognized by the neat fringes and tips. 
Secondary moult hasn't started yet in late-July, but is initiated in early-August.

Timing and strategy of body & covert moult in July

Wing-covert moult in 3cy and sub-adult michahellis vary between individuals. In general, conclusions based on average sub-adult michahellis are:

• Moult of head feathers is in progress, creating a "moth-eaten" appearance.
• sub-adult birds show plain adult-like grey scapulars. It's difficult to score missing feathers in the complete grey upper-parts, hence we have no scapular moult scores. In general, the upper lower scapulars look very fresh, the lower upper scapulars look older and worn and some of the scapulars in the lowest row seem to be missing in sub-adult michahellis by the end of July.
• The complete wing-covert moult is in progress throughout July. The ordinary moult strategy and sequence start in the outer median coverts, the innermost medians and innermost greater coverts, the upper tertial and the outer lower lesser coverts. Early July, the new median coverts are visible (first the outer median coverts), more so as the outer lower lesser coverts are dropped. By the end of July, the complete median covert bar is renewed, the lower lesser coverts are growing, the outer greater coverts are missing (exposing the secondaries underneath) and the innermost greater coverts have been renewed. In the tertials, the new upper two feathers are visible and the central tertials are dropped.

See Topography Section for explanation of feather tracts.

Tables: