Any flight shots, Ruud ?
The white leading edge to the hand of Black-headed Gull seems to be a dominant trait, and usually present in hybrids - though the sample of such hybrids is very small.
Derek Charles has photos of two hybrids here:
http://www.pbase.com/derekcharles/med_x ... ded_hybrid
Nico Geiregat had one last winter, and has posted the photo on this forum already, if I remember correctly:
http://waarnemingen.be/waarneming/view/46491919
Your fellow countryman Mark Hoekstein photographed one a few weeks ago:
http://waarneming.nl/waarneming/view/47922808
Here is an English bird from January 2009:
http://www.flickr.com/photos/richandrew ... 383383549/
(two more pics can be seen by clicking the left arrow in the set)
Photos of a Swedish bird can be seen in the
MeeuwNL mailinglist files, for those who are a member.
I have been sent a flight shot of a recent bird from Tienen (Vlaams-Brabant), Belgium, and I also have a flight shot of a Dutch bird (dated 7 April 2007, but I do not know the name of the photographer).
Furthermore, there is a painting of a German bird and some information in the
MeeuwNL mailinglist files, as well as sketches of and info on
a French bird.
A colour-ringed bird from Poland (red PEL3) is said to be a hybrid (but looks rather like a normal Black-headed Gull ?). A photo can be seen near the bottom of the following page:
http://home.scarlet.be/~pin02658/cr-MediterraGull.htm
A little more information on this bird
here.
That is all I know of. If other people have more photos, please share!
