Herring Gull 1cy (argentatus), December 15 2000, Westkapelle, the Netherlands (51.33N 03.25E).
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 an almost complete juvenile plumage by mid-December, 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 1cy michahellis (but which is of course much more advanced in the post-juvenile moult). Most argenteus show a deep-notched pattern along the fringe, although the white notched pattern is worn away by mid-December in most 1cy argenteus. The greater coverts in this individual do not show the familiar 'piano-key' pattern, as can be seen in many argenteus. Here in 1cy argentatus, the white bars are small and narrow, reaching the central shaft-streak only on the central greater coverts. The inner and 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 mid-December.
Birds as in the photo, with such deep brown feather-centres and this limited post-juvenile moult by December 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 1cy birds in December (bleached and worn feathers in December).