Lesser Black-backed Gull juveniles.
Movie by Mars Muusse.

"Young juveniles in the colony, only a few months old, may already show different patterned feathers in an otherwise complete juvenile plumage. These odd feathers typically look like second generation feathers. But they may in fact also be late acquired juvenile feathers, just different patterned."
A movie: in search for odd feathers...

(Flash 17 minutes, 720x576 1600 b/s, 44100 Hz).