Like all mammals, some amphibians and the rest of their reptilian relatives, birds engage in internal fertilization -- they transfer sperm from the male's body to the female's body so offspring can begin developing. While a few birds engage in intercourse, the males of most species lack penises and instead mate by simply pressing their cloacae together. This enables the male's sperm to penetrate the female's cloaca and ultimately ascend to reach the unfertilized ova waiting inside.
Because most birds lack a structure that serves as a conduit for sperm, their reproductive behavior differs from that of mammals and non-avian reptiles. For example, while many mammals and snakes copulate for extended periods, birds typically juxtapose their cloacae for only a few seconds per mating session. However, whereas mammals and reptiles usually require resting periods between mating sessions, birds may mate repeatedly over the course of an entire day.
Alternative Reproductive Structures
While the vast majority of bird species have lost their intromittent organs over evolutionary time, about 3 percent of living species' males do develop penises. Examples of such species include ducks, swans, ostriches and geese. Additionally, males of few species, notably chickens and quail, have small, penislike growths used for copulation. Cassowaries exhibit another reproductive system, as they use penislike structures during mating, but these appendages do not serve as conduits for sperm.
Peculiarities of Waterfowl
Female ducks and other waterfowl have spiral-shaped vaginas with numerous extensions and lateral outgrowths. Accordingly, males have corkscrewlike penises, which they hold internally when not in use. To mate, a male mounts a female and then -- within a fraction of a second -- extends his penis fully and penetrates the female's vagina. Often, such copulation is forced. Females have mechanisms for preventing nonconsensual fertilization. They can sequester sperm in a lateral outgrowth of the vagina to prevent it from contacting the ova.
In some cases, birds may be reluctant to mate with each other, which can sabotage the efforts of breeders. Veterinarians can artificially fertilize bird eggs by collecting a sperm sample and transferring the sample to the female's cloaca. Pet owners should not attempt to perform such tasks themselves.
Birds can produce eggs without fertilization; verify that an egg is fertile by holding it up to a strong light and looking for blood vessels, which indicate the presence of a living, developing embryo. Sometimes the embryo itself is visible as a small shadow or dark blob. You must be careful to use a bright lamp that produces relatively little heat to avoid harming the developing bird.