I'm pretty sure that the Caspian gull is the same as the one that has been found in Kluisbergen some months ago (about 10 km to the southwest), which also had some observations closer to Oudenaarde. Marks are: quite a lot of dark on the wingtips with restricted tongues and limited amount of white on the mirrors, even for a 4cy, marks on the bill and a tiny black marking on one or two central tail feathers.
During the winter of 2010-2011 I had some very similar observations of a 2nd winter at that time. It seems to me that this is the same bird, even though the bird was not very advanced in it's 2nd winter, reminding even of a 1st winter, and turned up almost adult-like now in it's 3rd winter with almost no markings in it's tail.
The 4cy Yelllow-legged gull has also been regular, now for it's 3rd consecutive winter near the ship lock.
The rareness of both species in this region allows to follow individuals, although not with a 100% certainty. Following them is time-consuming but interesting and great fun
Pictures are not of high quality, but I like them anyway.
http://vwgvap.waarnemingen.be/waarneming/view/63914316
http://vwgvap.waarnemingen.be/waarneming/view/63878489
Groeten uit Oudenaarde
Nico
