Mediterranean Gull adult 731 July 30 2001, Le Portel /Boulogne-sur-Mer, France (50.42N,1.34E).

In the top image: adult birds and a 3cy bird in the foreground, all moulting from summer to winter plumage in mid-summer. Note the plain white pattern of the outer primaries with just a black streak along the outer-web in the ringed bird: red 731, typical for full adult plumage. In the bottom images: one bird in complete summer plumage (with a complete black head and all primaries still present) and one bird moulting to winter plumage.

From late summer to September, a complete moult takes place, bringing birds into so-called "adult winter" plumage. The head pattern in this individual, red 731, still resembles summer plumage. After the complete moult, birds show a blackish mask behind the eye. The mantle, scapulars, tertials and all the wing-coverts have been replaced for plain pale grey feathers. 
The secondaries are completely white. The new adult primaries are shiny pearly-grey, flashing white at a distance. There is a black line along the outer-web of P10 (and sometimes on P9 as well). Secondaries, under-wing and tail are completely white. The bare part coloration: Iris brown, orbital ring red or orangey-red, the bill red or orangey-red with a black bill-band (and a yellowish tip in some birds) and the legs dull reddish or dark.

below: Mediterranean Gull adult, July 30 2001, Le Portel / Boulogne-sur-Mer, France >(50.42N,1.34E).

An adult, still in summer plumage with the head almost entirely black, except the eye crescents.

below: Mediterranean Gull adult, July 30 2001, Le Portel / Boulogne-sur-Mer, France (50.42N,1.34E).

Advanced moult to winter plumage, with only the outer primaries P8-P10 still old. The head shows a black mask behind the eye, connected between the nape and the crown and some loose spots.