noise in the darkness

One day of the winter, when I was riding back home from the downtown, I chose a different way of the common one. Suddenly, I realized of a strange noise in the dark. At the beginning I didn’t know what was it. But when I was getting closer I realized it was hundred of ducks! What?! In the middle of the winter, -20 º C and with all the lakes and rivers frozen. They were in a small lake of an stream on a neighbourhood. It seems that, I don’t know how, the lake is maintained unfrozen in order than these pets can survive the winter here without migrate.