Anas platyrhynchos  

It's the most popular duck in Poland. Female is gray and don't fit to beautifull colour male, but it's good feature to discern them.

My own observation I have done on the Bulwar Nadmorski (Seaside Bulwark) in Gdynia and on Patulskie Lake. Observing birds in city was easy, because ducks, which were fed by people were domesticate. I thought, that the observation on the lake was aimless and I stopped them, because afraid birds were flying away with noise. Ducks (from the lake) fly away in winter and come back in March, and that fed (from city) stay here. Cross duck, taking its food is diving not all time. It often puts only its head into the water.