In general, juvenile 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.