Here are some interesting internet links with a lot of information:

Bird research in general
SOVON - SOVON Vogelonderzoek Nederland
Trektellen - Trektellingen en ringvangsten
Javier Blasco Zumeta - Javier Blasco Zumeta’s site, Identification atlas

Ringing research in general
Vogeltrekstation - Dutch Centre for Avian Migration and Demography - A summary of all European colour ringing projects.
Crb-photoguide - A summary of photo's of colour rings. - Polish website with photo's from gulls and other birds.
Vogelringgroep 3e KP Vlieland - Results on captures by the Bird ringing group 3e KP Vlieland.

Ringing research gulls
Gull Research Group - Description of ringing research, also a lot of photo's on plumage and ages.
Black-headed Gulls on the Gull Research Group - Part of the above mentioned website
North Thames Gull Group - Studying the Gulls of Essex Landfill Tips - Black-headed Gull Research in Poland
Medgull website - Information on Mediterranean Gull colour ringing programs.
Black-headed Gull website - France website Black-headed Gull colour ring research
© Frank Majoor. Website update 03-02-2014.