Here is a cover from Finland with a stamp showing a pair of Mallards (Anas platyrhynchos).  This species of dabbling ducks can be found in wetlands in temperate and subtropical Europe, North Africa, Asia and America.  It was also introduced in Australia and New Zealand.  As you can see on the stamp the green-headed male is different from the brown and colour-lacking female.  This species feed on water plants, invertebrates, gastropods, worms etc.  It is migratory.

The stamp was issued in 2010.