General description:

By August, the number of migrating Herring Gull and Lesser Black-backed Gull increase dramatically in NW France, but 3cy Yellow-legged Gull can still be found commonly in the vicinity of Etaples and Boulogne-sur-Mer (see Le Portel).
The general appearance of 3cy michahellis is rather straightforward. Most birds can be recognized by the plain grey upper-parts and coverts, combined with some retained barred wing-coverts, especially in the carpal edge. The bare parts are typically a red orbital ring, a black band on the bill and yellow legs. An example of the average August 3cy michahellis can be found in the sixth image, an individual with adult-like upper-parts, some barred wing-coverts in the carpal edge, some new white tail-feathers and the new primaries just visible beneath the tertials. 
In the surveys of the last days of July 2001 3cy michahellis showed an average primary moult score of 8.5 remaining old primaries. By the last week of August, 3cy michahellis may show two, one or no old primaries. The table below shows figures for July and September.
For details on differences between michahellis and cachinnans, see e.g. the article by R. Klein & D. Gruber in Limicola, April 1997.

Moult stage and pace of remiges and rectrices:

The complete moult started by mid-May, when 3cy birds dropped the innermost primary P1. By early July, the average score has strongly increased: P7-P10 are still old second generation and the inner primaries are new. Since the tips of P5 and P6 normally extend beyond the tertials (especially the abraded July tertials), the moult gap of missing P5 and P6 is obvious. 
3cy michahellis continue shedding primaries throughout the month, and by the end of July, the average primary moult score for remaining primaries is 8.5. With this score, 3cy birds are about two primaries behind compared to 2cy birds (which have a score of 9.2: 45% of 2cy have shed all primaries by late July). 
As in adults, the gap between full-grown primaries and remaining old primaries is in general about three feathers, or slightly more as the table in the July section shows. This large moult gap is typical for michahellis, and not found in e.g. Dutch argenteus.
By early August, 3cy michahellis has an average score P6 fully grown, so the majority of resting birds shows new primaries in the folded wing. This score increases gradually during the month. We have no late-August or early-September scores, but by the third decade of September the primary moult score exceeded 8.0. The score for the end of August must be about P7.

By the time P4 is fully grown, 2cy birds normally start the complete moult in secondaries and rectrices. The ordinary moult strategy in 3cy birds may be slightly later (starting when P5 is fully grown. Note however that individual variation may exist, particularly in 3cy michahellis rectrices moult as this moult may follow an irregular pattern in earlier months. The partial spring moult has finished in June and a small minority of 3cy June michahellis have included some rectrices in this partial moult. Hence, it's not uncommon to find 3cy birds showing plain white tail-feathers, in some birds creating a 'blocked' tail pattern already, prior to the complete moult. 
On average, the common sequence is centrifugal moult: starting with the inner tail-feathers and continuing symmetrically outwards to both outer feathers. The moult in the rectrices is still in progress by late-August and the new new third generation tail-feathers normally show some black markings.

Secondary moult starts in early August. The images on the July page showing birds with open wings illustrate that all outer secondaries are still present. By early August, the outermost secondaries (S1 & S2, at the division between secondaries and primaries) are normally dropped individually, but the central secondaries are shed almost as in small groups, creating large gaps of missing secondaries, nicely illustrated by this flying 2cy bird and this stretching 3cy August michahellis. 
The innermost secondaries are still second generation. Since many birds moult the inner greater coverts, the secondaries underneath are exposed, nicely demonstrated by this bird.

Timing and strategy of body & covert moult in August

Wing-covert moult in 3cy and sub-adult michahellis vary between individuals. See e.g. 365D, showing extensive moult in the wing-coverts (the sequence is mentioned in the accompanying text). Such birds contrast strongly with e.g. this bird, which has almost all the wing-coverts old.

In general, conclusions based on average 3cy michahellis are:

• Moult of head feathers is in progress, but almost finished in advanced individuals, creating a streaked head. See here (streaking around the eye) and here (streaking around the eye and in the hind-neck).
• Average 3cy birds show plain adult-like grey scapulars from July onwards. It's difficult to score missing feathers in the complete grey upper-parts, hence we have no scapular moult scores. In general, the scapulars in the lowest row look very fresh, without abrasion at the fringes and lacking the faded grey tone, so scapular moult may be near its end in 3cy michahellis by the end of July and early August. The odd individual may still be found missing rear lower scapulars, normally the once moulted last in line.
  In some individuals, the scapular region contains some old and abraded feathers (see here, in the lowest scapulars), sometimes showing a brown hue, others are bleached white-grey with only a slightly darker shaft-streak. However, most birds have the entire back adult-like grey.
• The complete wing-covert moult is in progress throughout July and August. 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. By August, the median and lower lesser coverts have been replaced and 3cy birds are missing the outer greater coverts and some lesser coverts. By the end of August, the outer greater coverts have been replaced as well and the gap in the greater coverts will be closed at greater coverts #7-#8 in birds following the ordinary moult strategy, see e.g. here (missing inner greaters) and here (closing the gap at #7-#8). 
  By late-August, the most advanced birds have replaced all the wing-coverts and may start again a new additional cycles of wing-covert moult: the partial wing-covert moult in autumn. They normally start at the oldest wing-coverts, i.e. the median coverts and lower lesser coverts. See e.g. here.
• In the tertials, the upper 3-4 feathers have been replaced by the end of August and the lower two tertials are missing. During the month of September, 3cy michahellis moult the complete secondary row and tertials (which actually belong to the secondaries). The gap in the secondaries will be closed at about #18.

See Topography Section for explanation of feather tracts.

Tables:

The surveys were done at resting and preening 3cy sub-adults (with at least no white mirror at P10 and at least some obvious second summer feathers).

 

Primary moult score of 3cy michahellis Yellow-legged Gull, at Etaples, NW France (50.43N, 01.37E) on August 09 2003.
.	P5 fg	P6 fg	P7 fg	? fg	n:
P8-P10	3	-	-	1	4
P9-P10	1	8	-	1	10
P10 old	-	5	-	-	5
no old P	-	-	3	-	3
n:	4	13	3	2	22
n = 22; m old P = 1.7; SD old P = 0.95. Group of 328 michahellis present: 28 juveniles, 37 2cy, 35 3cy, 218 (sub)adults. P8-P10 = P8-P10 are still old and present. ? fg = fully grown not known. Fully grown primary score most probably P6 or less. fg = fully grown. In our field surveys, a primary is 'fully grown' when it exceeds the previous feather in the folded wing. Actually,  such 'fully grown' feathers may have to grow a little, but under field circumstances, it's very difficult to find out if the waxy sheath, which is the standard measurement for fully grown primaries, is still present.

 

Primary moult score of 3cy michahellis Yellow-legged Gull, at Etaples, NW France (50.43N, 01.37E) on August 11 2003.
.	P5 fg	P6 fg	P7 fg	P8 fg	n:
P8-P10	5	-	-	-	5
P9-P10	3	24	-	-	27
P10 old	-	10	6	-	16
no old P	-	-	2	-	2
n:	8	34	8	-	50
n = 50; m new P = 6.0; SD new P = 0.57; m old P = 1.7; SD old P = 0.70. Further comments / notes: see previous table.