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).

When birds arrive at the wintering grounds or at the stop-over sites along their migration route, there is often a visible divide between fresh inner primaries (3rd generation) and older outer primaries (still 2nd generation).