Male ducks, which are called drakes, fertilize eggs through copulation with female ducks, known as hens. Copulation typically occurs after a courtship ritual during mating season, which is in early spring. Drakes must mate with hens once every four to five days to ensure fertilization.
According to PBS.org, ducks practice internal fertilization, which means the process takes place inside the hen's reproductive tract. This protects the developing egg.
According to SouthFloridaMuscovyDucks.com, drakes engage in courtship rituals that include head bobbing, exaggerated feeding movements, preening and counter-movements. They also emit calls particular to courtship and stand in specific postures, such as holding their head and tail up.
According to About.com, reproductive organs of birds differ from mammals'. When ducks are ready to mate, their reproductive organs swell within a bodily exit called a cloaca. Both drakes and hens have cloacas. Once a hen has indicated her acceptance of a drake, the drake balances on top of the hen, and copulation occurs. An extension of the cloacal wall via lymphatic fluid forms a penis, which allows ducks to mate in the water without the sperm washing away.
The egg and sperm combine inside the hen to create a fertilized egg, a zygote, which is coated in a protective shell. The hen lays the egg and sits on it until it hatches, which is called brooding. Drakes generally abandon the hen to her brooding.