General description:

3cy michahellis in September can be recognized by the plain grey upper-parts and coverts, often combined with some retained barred wing-coverts, especially in the carpal edge and the lower tertials. Most birds have finished the complete moult in the under-parts and head, now showing much streaking in the crown, nape and ear-coverts. The bare parts are typically less bright coloured than last summer or as in adults, now showing a brown-red orbital ring, often a dark hue in the iris, much black on the bill and still straw-yellow legs. An example of the average September 3cy michahellis can be found in the first image, an individual ringed 074S with adult-like upper-parts, some barred wing-coverts in the outer greater coverts, a pinkish base of the bill and much speckling in the iris. This bird was ringed in summer 2000 in S France. Although 074S shows some clear 3cy features, some birds may be very advanced in moult, much resembling 4cy birds, as is demonstrated by 255S, ringed on the same location as 074S. A very typical 3cy michahellis can be found in the October Section: this bird photographed on October 01 2001.
The table below shows the average primary moult score for 3cy michahellis in September: about 9.0 at the end of the month. So, the average bird only has to grow one primary to finish the complete moult, which started last May.
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 mid-May, when 3cy birds dropped the innermost primary P1. By early July, the average score has strongly increased: only P7-P10 are still old second generation and the inner primaries are new. By August 01, the average primary moult score is about P5. With this score, 3cy birds are two primaries behind compared to 2cy birds . 
As in adults, the gap between full-grown primaries and remaining old primaries is about three feathers in general, 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.
The score for late-September is 9.2 in two surveys totaling 56 individuals in NW France.

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, 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 completed in September. New feathers are white with various amount of black markings. Some individuals have a clean white tail. The new feathers can be recognised by the fresh fringes and tips.

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 secondary moult finishes with the replacement of the inner secondaries (about S18), close to the tertials (which in fact are secondaries as well). The completion of the secondary moult coexists with moment that P8 is fully-grown. This is late-September in most individuals.

Complete moult vs. partial moult

In the complete moult during the summer, 3cy michahellis moult all the scapulars, wing-coverts, tertials and flight-feathers. The complete moult started in May, when P1 was dropped and the complete moult will be finished when P10 will been replaced in October. By August, the complete moult in the wing-coverts is finished with replacement of the last central greater coverts and finally the feathers in the carpal edge (outer lesser coverts). As those last lesser coverts are replaced, 3cy michahellis start the partial autumn moult. In this partial moult some wing-coverts and tertials are included. It starts again with the upper tertials and outer median coverts and is nicely illustrated by this individual from October 01 moulting P8. The outer median coverts are clearly missing.
3cy michahellis show a much larger individual variation in moult stage than 2cy birds. (For instance: the same individual from October 01 with P7 fully-grown, while the average score for late-September is 9.2). So its hard to generalize on partial moult stage in 3cy in September, and it may be strongly related to the primary moult stage of an individual.
However, partial moult may start by the last week of August in advanced birds. By late-September, the innermost greater, median and lower lesser coverts and the upper tertials have been replaced in the partial moult.

The partial moult in 2cy michahellis is often more obvious and explained in the 2cy michahellis section. 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).