The Ruddy Duck is a diving duck species – males can be easily identified by their bright blue beak. They were imported to Britain to be bred in the 1940s and it is thought that their escape led to rapid expansion of the population. Culling of ruddy ducks began in 1999 and has greatly reduced their numbers. They are thought to have little impact on British ecosystems but their prevalent population here was a source for the rest of Europe. White headed ducks in Spain were at risk of hybridisation from ruddy ducks and this was the reason for the cull. They were recorded a couple of times in the lower catchment of the River Thame in 1985 and 2002.