Duck eggs are fertilized through copulation between a male and female duck. Although male ducks are an aggressive species when it comes to mating, female ducks have adapted special defenses against the drake's constant wooing.
Eggs laid in an all-female brace of ducks will never be fertile: A male duck -- also called a drake -- must be present and mate with the hens in order for eggs to be fertilized. However, the actual fertilization process is tricky.
You see, with ducks, mating is a rather violent affair. Drakes are one of 3 percent of bird species that have a penis instead of a cloaca, and when they mate with a female duck they physically coerce them into copulation. Most of the time, this act occurs against the female duck's will. Male ducks will force their corkscrew-shaped penis into their chosen female companion, in an attempt to fertilize her eggs.
Drakes have evolved some fascinating adaptations, given their rather unromantic approach to fatherhood. For one, a duck's penis grows every single year, and the more competitive he is in mating, the longer it grows.
An Interesting Twist
Duck hens usually have no choice in the matter when a male duck decides to mate; she often cannot physically move away from her captor. The hen is not left utterly defenseless, however, as she too has co-evolved.
A hen's vagina is also corkscrew-shaped, but in the opposite direction of a male duck's phallus. Additionally, her vagina has evolved to sport numerous "dead end" pockets, meaning that with a little control, she can make sure her unwanted suitor deposits his sperm not in her avian sperm storage, but in an empty pouch, to be expelled later. This gives her more control to select the appropriate father of her ducklings than it appears.
Saved for Later
Fertilization occurs during the process of laying an egg. As an ovum moves down her oviduct and begins to form into an egg, the duck hen mixes in a little sperm from her sperm storage. The eggshell then forms around the ovum, yolk and sperm, and creates the egg we recognize.
She will do this every day up to two weeks, laying an egg each day. Once the entire clutch of eggs has been laid, the new mother will begin to incubate them, until they hatch. During this time, you can check to see if the eggs have been fertilized with a process called "candling."
Anyone in There?
In a method that hasn't changed for hundreds of years, candling is simply when you shine a bright light behind an incubating egg. By the fifth day of incubation, a live egg will display reddish veins across the yolk. Around the twelfth day, the distinct shape of the growing duckling -- and possibly some movement -- can be seen.
Before breeding your ducks or candling eggs, always check with a veterinarian familiar with poultry and livestock. They can give the best guidance on how to keep your ducks safe, healthy and happy.