I've done a fair bit of swotting up on the subject this evening, ranging from PAC's adult on Flores in December 2005 in Dutch Birding to the more usual references such as Gulls of the Americas and the standard DB American Herring Gull reference. And also a few internet searches, including the regular bird in Ireland (http://www.birdguides.com/iris/pictures.asp?v=1&f=195040).
|primaries show grey pinching into the black tips - 'bayonets' to P7 and P8?|
|pale underside tongue to P10 long and rectangular|
|tricky to get exact primaries here due to photo quality. P5 seems to show that black 'W' over both webs...|