Slaty-backed Gull (schistisagus) / オオセグロカモメ / 큰재갈매기

(last update: December 2015)

Slaty-backed Gull (schistisagus) 4th basic moult / 4CY September

Identification of this age class is difficult. Birds are supposed to be in 4CY plumage when they combine two aspects; a. new flight feathers are adult-like, without immature markings, and b. old flight feathers, which are (heavily) worn, are old 3rd gen feathers (adult-like patterns, like bold white tips on old secondaries, limited black in tail and P10 sometimes with distinct mirror).
Separation between 4CY and retarded 5CY birds in September is tricky or unreliable.