Typical juvenile nominate fuscus are hard to describe as the variation in colour and shape is extensive, but on average a few points may indicate specific identification. Note however that single individuals from graellsii, intermedius and fuscus are almost impossible to tell apart as there exists a large overlap. For comparison, a juvenile intermedius from Götenborg, SW Sweden is added at the bottom. Images of ringed graellsii and unidentified graellsii / intermedius can be found in the Lesser Black-backed Gull sections of August, September, October and November-December.

Colour and contrast

As early as August, many nominate fuscus are remarkable contrastingly patterned. The centres of the scapulars and wing-coverts are very dark brown, almost blackish brown. The fringes are very pale and quickly lose the brown hue characteristically found in very fresh fledged juveniles. The fringes are rather pale, almost white. The average graellsii and intermedius in the Dutch colonies (Maasvlakte, near Rotterdam) seem to be warmer coloured, although individuals with very pale fringes and dark brown centred scapulars can be found in Dutch colonies as well (see e.g. there). On the other hand, individuals with warm brown fringes can be found in Finland, as the first image shows. This bird was still begging at a parent at the Tampere landfill and typically these warm brown individuals may turn more contrastingly patterned once the fringes bleach after several weeks. Some juveniles remain warmer brown during the month of September. Nevertheless, the average picture hold with nominate fuscus more contrastingly patterned and the graellsii / intermedius juveniles on average warmer brown, especially on the fringes of scapulars and wing-coverts, but also on the nape, ear-coverts and belly.

Jonsson, in his article in Birding World volume 11 no 8 (1998) estimates a 10% of juvenile fuscus to show very pale fringes "the light areas sometimes appear 'over-exposed' so that the belly and head can be almost white" and he classifies these juveniles as 'light morph birds'. The other 90% belong to the average group and some appear dark brown, almost lacking any pale fringes in the greater coverts. 

Shape and expression

As with the adult nominate fuscus, juveniles appear more delicately built than their western relatives intermedius and surely graellsii; fuscus being small, long-winged and thin-billed. Especially the obvious P10 projection, well over P9, attracts attention but dealing with a single bird, there may be no particular characteristic to identify a juvenile fuscus from intermedius.

Post-juvenile moult in juvenile fuscus

As can be seen in the pictures, most birds show a complete juvenile plumage by early August. Numbers are too low to draw any conclusions about the post-juvenile moult, but at least a few birds started the post-juvenile moult as early as August 07 at the Tampere landfill, in SW Finland (see e.g. this bird and CHR5). Jonsson (in his article in Birding World) mentions that post-juvenile moult may start in August in some individuals. On the other hand, juvenile fuscus may arrive at the African wintering grounds, still in complete juvenile plumage by late September to December according to Jonsson. Part of the explanation could be related to the large breeding area in Scandinavia, from the southern latitudes, Baltic region through central Norway all the way north to northern Norway and east to the White Sea. 
Jonsson observed juvenile fuscus mainly at Gotland SW Sweden, where most birds are growing second generation scapulars and mantle feathers by mid September. New scapulars may show a distinct anchor shape, as can be found commonly in graellsii and intermedius (see again the sections of September and October, other juvenile fuscus replace the juvenile feathers for plain dark slate-grey feathers with a broad blackish wedge-shaped centre. Such plain patterns can only rarely be found in intermedius by early autumn. Below, tables are presented to show the post-juvenile moult timing of Dutch intergrades in colonies in SW Netherlands.

Tables