Last wednesday, 20/10/2010 at Peniche - Portugal, I spotted what it seemed a small GBBG, with a large round head, small eye and strong bill than the YLG and LBBG that were also present.
But darker overall impression, specially barred uppertail coverts and rump.....made me hesitate. YLG can show large Head profile and bill, but eyes are larger, giving a different look to the head.
Hello Cristrian, odd YLG came to my mind also, in fact, plumage shouts for YLG..... but Ruuds explanation, how salt glands can change head profile, explains it all.
Other YLG present, show at some level the same caracteristic....