At the right time of year you wouldn't look twice at an adult black headed gull in summer plumage, but in late November its a different kettle of fish. This bird was feeding with about 200 other gulls in a howling gale on a playing field south of Pontypool at lunchtime today. I was intrigued as none of the other gulls came anywhere close to matching this birds summer plumage.
My copy of Gulls of Europe, Asia and North America (Klaus Malling Olsen and Hans Larsson 2002) provided an answer, it states. Adult with dark hood in midwinter rare but regular. Several birds showed traces of darker eye spot, others had fresh hood Nov-Dec, indicating that hood atypically may have been developed during autumn moult or as very early moult into summer; although mostly incomplete into mid-Jan. This has especially proven to be the case following a series of very mild autumns and winters.
It goes on to say. Earliest with winter head mid Jul (small minority mid-Jun probably failed breeders); by late Aug;90% have winter head. Small minority retain summer head to early Oct, exceptionally into winter.