Most bird species do not possess an external penis; only about 3% of nearly 10,000 species worldwide have a functional external reproductive organ. Instead, the vast majority of male birds rely on the cloaca, a multipurpose opening for digestive, urinary, and reproductive functions. While most birds lack a penis, notable exceptions exist. Certain groups, particularly waterfowl like ducks, geese, and swans, along with some flightless birds such as ostriches, do have a phallus.
Avian Reproductive Anatomy
The cloaca is a common chamber and outlet found in birds, amphibians, reptiles, and some fish. It serves as the exit point for digestive, urinary, and reproductive tracts, used for expelling waste and transferring sperm or eggs. This multi-use design is an efficient anatomical feature, allowing birds to maintain a lighter body weight beneficial for flight. Internally, the cloaca is divided into three sections: the coprodeum for feces, the urodeum for urine and genital products, and the proctodeum for storing waste before expulsion.
For most bird species, reproduction occurs through a brief physical contact known as a “cloacal kiss.” During breeding season, the cloacal regions of both male and female birds swell. Mating involves the male mounting the female, with their cloacas briefly touching, often for less than a second, to transfer sperm. This rapid transfer helps birds avoid predators and minimizes vulnerability during copulation. The female then draws the sperm into her reproductive tract for fertilization.
Birds With a Phallus
Some bird species possess a phallus, the scientific term for the avian penis, unlike most birds that reproduce via the cloacal kiss. This organ is found in groups like waterfowl (ducks, geese, swans) and some ratites (ostriches, emus, rheas). The avian phallus is distinct from the mammalian penis, having evolved independently, and is not considered homologous.
The phallus is typically stored internally within the cloaca and becomes everted or extended for mating through hydraulic pressure, rather than blood flow as in mammals. In some species, such as ducks, the phallus exhibits a unique corkscrew shape, which can be quite long, even reaching lengths greater than the bird’s own body. This structure is solely for sperm delivery and is not involved in urination. Its presence is often linked to aquatic mating environments or intense sexual selection pressures.
Evolution of Avian Reproductive Organs
All birds initially develop a genital tubercle in the embryo, the precursor to a penis. In most bird species, however, a gene called Bone Morphogenetic Protein 4 (Bmp4) activates, triggering programmed cell death (apoptosis) in the developing tubercle. This process causes the regression and eventual loss of the external phallus, resulting in a non-functional or absent structure in adult birds like chickens and quail.
Conversely, in species such as ducks, Bmp4 gene activity is suppressed or switched off, allowing the genital tubercle to continue developing into a functional phallus. Scientists propose several theories for why most birds lost this organ. One idea suggests the absence of a phallus might be an unintended consequence of Bmp4 influencing other developmental processes. Another hypothesis involves female choice, where females may have favored males without an external phallus, leading to its reduction over time.
Sexual conflict drives the evolution of avian reproductive organs. In species with intromittent organs, like waterfowl, coevolutionary “arms races” occur, where male phallus morphology and female reproductive tract complexity evolve in response. For instance, the corkscrew-shaped duck phallus often accompanies a female vagina that coils in the opposite direction, potentially allowing females some control over fertilization. A phallus in waterfowl may also be advantageous for mating in aquatic environments where cloacal kiss sperm transfer is less efficient.