www.gull-research.org

http://waarneming.nl/waarneming/view/62052770

The observer is confident it concerns a Glaucous but I don't see how to exclude a larger Iceland like this one
http://waarneming.nl/waarneming/view/61817625

I don't see these species often enough to confidently ID by jizz but hopefully someone on this forum can

Hi Ruud,

on brief and/or distant views, it can be really difficult to separate these two species in flight.
However, when the bill can clearly be seen, as here, identification is much easier.
With such a small and greenish bill, I think there can be little doubt that this is an Iceland Gull, not Glaucous...

Thanks Peter, apparently the observer has come to the same conclusion!