Mew Gull heinei? K48 adult, January 07 2006, Enghaveparken, Vesterbro, København, Denmark. Picture: Jørgen Witved.
Much black in wing, especially P8-P10, may indicate ssp heinei.
In full adult plumage, open-wing images can be checked on several subbtle points, which, in combination, may result in a fairly different wingtip pattern for heinei: in general showing more pigmentation. Also, the grey upperparts may be distinctly darker than canus and lack their blue tone. Details in the wingtip are key.
(i) heinei more often show a deep black full subterminal band on P5; where canus normally has a thin, uneven or broken band, or black only on one web. Sometimes, full adult heinei may show a black spot on the outer-web of P4;
(ii) in heinei, black pigmentation extends all the way to the primary coverts on P8, where canus only has this black reaching 50-75% of the primary;
(iii) heinei shows extensive black on P7 (more than 80% of the way to the primary coverts); while canus has mostly less than 50%;
(iv) a long black 'bayonet' on the outer web of P6 extends for approximately 65% of the way to the primary coverts in heinei , while the maximum in Chris's canus sample is 53%, with most having black only 20-40%.
The overall result of these details is a wingtip that has a lot more black and less white than canus.