In general, fuscus is described as a small Lesser Black-backed Gull (LBBG). Compared to graellsii and intermedius it has more contrast in upper-parts and wing-coverts. Dark patterns look darker and pale patterns look almost white, resulting in an "over-exposed" juvenile bird. This character, combined with the elongated wing, slender bill, small size of the bird and the peaked crown with the highest point behind the eye should give some clues to distinguish juvenile fuscus from other LBBG juveniles (graellsii & intermedius).

Before departure to wintering grounds, some juvenile fuscus moult mantle and scapulars to second generation feathers. Migration may start early, with juvenile birds arriving in Africa by late September, but most arrive in October, in some cases in complete juvenile plumage. Normally a rapid moult starts quickly after arrival: scapulars, coverts, secondaries and primaries may be included in the post-juvenile moult. By April, some 2cy birds migrate north to the breeding areas. On arrival, the wing may look anything from only a few new primaries moulted to completely 2nd generation primaries. Normally, moult is arrested during migration and a clear division is visible between old juvenile and fresh 2nd generation primaries.

Primary moult

The moult sequence in nominate Lesser Black-backed Gull Larus fuscus fuscus was already described by Stresemann & Stresemann in 1966 in Journal für Ornithologie. They described a periodische Staffelmauser, a step-wise moult. Normally, large white-headed gulls in Europe start replacement of the remiges during the breeding season and finish this moult in autumn, prior to their migration.

Stresemann & Stresemann described fuscus as an exception as it doesn't start the remiges moult before arriving at the wintering grounds in Africa and the Mediterranean. For a long-distance migrant like fuscus, this seems to be a suitable strategy. This pattern can also be found in long-distance migrants like Terns (Sternidae).

Stresemann & Stresemann examined skin collections and failed to find any adult L. f. fuscus originating from the breeding or autumn migratory range that was in active primary moult. Therefore they presumed the entire moult stage takes place on the wintering grounds. Adult birds on migration had been collected (Rossitten Bird Observatory, Russia) and these birds confirmed the theses: they must have suspended moult, as Heinroth (1928, Die Vögel Mitteleuropas) concluded: adult fuscus doesn't show active primary moult on migration. They either suspend moult or do not commence moult before arrival on the wintering grounds.

However, there are a few museum skins that do show adult fuscus in primary moult: the Zoological Museum of Helsinki (not visited by the Stresemanns in 1966) has 21 skins of summer / early autumn birds: 8 are showing moult in the primaries. The museums of Copenhagen and Tring have 10 skins in collecting from the same period: one is showing active primary moult.

The last decades, the general idea about the renewal of the primaries in adult fuscus has been changed slightly. Field research showed that a small majority (up to 60% in southern Sweden) of the adult fuscus have started to replace the inner one or two primaries on the breeding ground by late August. This primary moult is suspended until arrival at the tropical wintering grounds. Fuscus complete their primary moult in February and March, just before leaving again to the breeding grounds in Scandinavia and northern Russia. By March, the inner primaries may in some birds included in the primary moult, in a second wave. This seems to be a common feature in sub-adult birds.