Herring Gull 2cy (argentatus), January 04 2002 , Boulogne/Mer, France (50.42N,1.34E).

Argentatus is a breeding species of northern latitudes in Scandinavia, and as can be expected, the moult to second generation feathers is delayed, compared to south-western breeding argenteus. This individual still shows a complete juvenile plumage by January, where argenteus would show second generation mantle-feathers and upper scapulars. 
Argentatus quite often show narrow fringes combined with extensive dark centres in the tertials and pretty dark outer greater coverts, somewhat reminiscent of michahellis (but michahellis is of course much more advanced in the post-juvenile autumn moult). Most argenteus show a deep-notched pattern along the fringe, although the white notched pattern is worn away by January in most 2cy argenteus. The greater coverts in this individual do not show the familiar 'piano-key' pattern, as can be seen in many argenteus. Here in this 2cy argentatus, the white bars are small and narrow, reaching the central shaft-streak only on the central greater coverts. The outermost greater coverts are predominantly brown, the centre reaching to most of the feather's edge.
The powerful and bulky appearance, together with the contrasting deep brown upper-parts strongly advocate northern argentatus. Argentatus from the northernmost populations remain in Scandinavia for most of the autumn, and this factor, together with the later hatching date, may be one of the reasons the juvenile feathers are not as bleached as in argenteus by January. 
Birds as in the photo, with such limited post-juvenile moult by January are definitely not argenteus and belong to argentatus. However, remember that argentatus also breed as south as the Baltic states and southern Finland, adopting very much the same breeding habits as argenteus with similar moult scores (starting the post-juvenile moult in August) and wear in the lower scapulars, inner wing-coverts and tertials in 2cy birds in January (bleached and worn feathers in January).