bird 5 - presumed hybrid Slaty-backed x Vega Gull,
adult, Inubou, Japan, 9 March 2012.
Even though this bird was seen only briefly,
flying past the cliffs, I think it can be readily
identified by its blackish-grey upperwings suggestive
of Slaty-backed Gull and the very pronounced contrast
between black outermost primary with rest of underwing (as in Vega).