http://www.euring.org/
   
  http://www.cr-birding.be/
   
  http://pt-ducks.naturlink.pt/
   
  http://www.holmer.nl/Colourrings.htm
   
  http://www.juvaste.fi/
   
  http://www.lpo-anjou.org/baguage/code_BTO.htm
   
  http://www.gull-research.org/