Gulls in NW Europe
(last update: September 25, 2011
Herring Gull sub-adult, January 23 2013, Katwijk, the Netherlands.
4CY and 5CY birds difficult to separate in many cases, but a single small mirror on P10, much dark in secondaries & tail, and extensive brown coverts is generally associated with 4cy birds.
Birds are probably 5CY, when only vestiges of black are present in greater primary coverts; otherwise they show a white tail, unmarked secondaries and sharply defined wing-tip pattern. Still, exact ageing remains difficult in unringed birds, and argenteus may develop a more advanced plumage than nominate argentatus, so age of unringed birds probably best left unclassified as "sub-adult".
Bird below looks good for local 5CY argenteus.
