Do Male Birds Have a Penis? The Biological Answer

Most male birds do not have a penis. Roughly 97 percent of bird species mate by briefly pressing their cloacal openings together in what ornithologists call a “cloacal kiss,” with no intromittent organ involved at all. The exceptions, however, are spectacular. Ducks, geese, swans, ostriches, emus, and a handful of other groups retain penises that can be surprisingly elaborate. The story of why some birds kept this organ while most lost it involves embryonic cell death, an ancient shared ancestor with crocodilians, and one of the more dramatic examples of an evolutionary arms race between the sexes.

Which Birds Still Have a Penis

The birds that possess a true intromittent phallus fall into a few distinct groups. Waterfowl, the order Anseriformes, are the most well-known. Ducks, geese, and swans all have penises, and in some species these organs are remarkably long relative to body size. Ratites, the large flightless birds like ostriches, emus, cassowaries, and rheas, also retain a functional penis. Tinamous, a group of ground-dwelling birds from Central and South America that are closely related to ratites, have phalluses too, though some tinamou species have reduced them to a non-intromittent nub.¹

Among the chicken-like birds (Galliformes), the picture is more nuanced. Chickens, turkeys, quail, and most of their relatives lack a functional penis. But some members of the megapode family, the mound-building birds of Australasia, retain a small non-intromittent phallus. At least one megapode genus has lost the phallus entirely, representing yet another independent evolutionary reduction.¹

Then there is the vast superorder Neoaves, which includes songbirds, raptors, parrots, pigeons, woodpeckers, and the overwhelming majority of bird species you are likely to encounter. The common ancestor of all these birds lost the phallus entirely. None of the roughly 10,000 species in Neoaves has one.

How Most Birds Mate Without One

For the majority of bird species, mating involves a brief and often awkward-looking act. The male mounts the female’s back, and both birds twist their tails so that their cloacal openings, the single all-purpose vent used for excretion and reproduction, meet briefly. Sperm transfers in a fraction of a second. The whole encounter often lasts just a second or two, and to a casual observer it can be hard to tell that mating happened at all.

Some penis-less species have evolved modifications to improve the efficiency of this system. Male fairy-wrens and their relatives in Australia, for example, have a small muscular protrusion at the tip of their cloaca. Research on several species in this family found that this cloacal tip is a muscular structure made of longitudinal muscle fibers, connective tissue, and a toughened outer layer. Species facing more intense sperm competition, as inferred from relative testis size, had longer cloacal tips.²

Other species take a different approach. The alpine accentor, a small Eurasian songbird with a notably promiscuous mating system, has a large cloacal protuberance whose internal structure includes a highly convoluted extension of the sperm duct. This structure stores and maintains sperm, with different regions apparently specialized for storage versus ejaculation.³ These cloacal modifications are not penises. They do not enter the female’s body. But they show that even in the absence of an intromittent organ, sexual selection has found ways to shape male reproductive anatomy.

The Duck Penis and Its Explosive Mechanics

If you have heard anything about bird penises, it is probably about ducks. Duck penises are long, corkscrew-shaped, and they evert in an explosive fashion. Work on the muscovy duck measured a penis about 20 centimeters long that everted in roughly a third of a second, reaching speeds of about 1.6 meters per second.⁴ Unlike most vertebrate penises, the duck penis does not become rigid when erect. Its collagen matrix is very thin and lacks the kind of structural arrangement that would provide stiffness, so the organ remains flexible even when fully everted.

The duck penis is also unusual in how it achieves erection. In most vertebrates, the penis fills with blood. In birds that have a penis, the mechanism is lymphatic rather than vascular. Lymph fluid fills sinuses within the organ to extend it.⁵ This represents a major evolutionary shift. The ancestor birds share with crocodilians almost certainly used a blood-based erection system, as crocodilians still do. At some point in the bird lineage, lymph took over that role.

The counterclockwise spiral shape of the duck penis is functionally important. When researchers tested eversion in glass tubes of different shapes, the penis everted successfully through a straight tube and a counterclockwise spiral tube matching its own shape, but was significantly less successful when the tube spiraled in the opposite direction or bent at a sharp angle.⁶ That finding makes more sense once you know what the female’s reproductive tract looks like.

The Arms Race Between Duck Genitalia

Female ducks have vaginal tracts that are, by vertebrate standards, bizarrely complex. In species where males frequently attempt forced copulations, females have evolved vaginas with clockwise spirals (opposite to the male’s counterclockwise spiral), dead-end pouches, and sharp bends. These structures appear to make it harder for an unwanted male to achieve full intromission and deposit sperm effectively.

A comparative study across 16 waterfowl species found that the degree of vaginal elaboration was positively correlated with phallus length after accounting for evolutionary relationships between species. In other words, species where males had longer penises also had females with more complex vaginal anatomy.⁷ The researchers concluded that this co-evolution was driven by sexual selection and that females had evolved what amounts to a hidden anatomical mechanism for choosing which males successfully fertilize their eggs.

The broader interpretation is that female waterfowl genital elaborations function to reinforce female mate preference and preserve the benefits of choosing a high-quality partner, rather than simply to avoid physical injury from forced mating.⁸ This distinction matters because it shifts the narrative from “females are defending themselves” to “females are exercising control over paternity.” The glass-tube eversion experiments described earlier essentially confirmed the mechanical basis for this: a female duck’s vaginal geometry genuinely impedes a male penis that spirals in the wrong direction.

How Most Birds Lost the Penis in the First Place

The best evidence suggests that the common ancestor of all birds had a penis. So did the common ancestor birds share with crocodilians. Studies of reptilian and bird development point to a single midline phallus with a sperm groove being the ancestral condition for all amniotes, the group that includes reptiles, birds, and mammals.⁹ Losing the penis is a derived condition, something that happened later in bird evolution, not the starting point.

The developmental mechanism behind the loss was worked out by studying chicken and duck embryos side by side. Both species start building a genital tubercle, the embryonic precursor to a penis, in early development. In ducks, that tubercle keeps growing and becomes a functional phallus. In chickens, the tubercle initially develops but then undergoes programmed cell death, causing the structure to shrink away before hatching.¹⁰

The molecular signal responsible is a protein called Bmp4. In chicken embryos, Bmp4 becomes active in the genital tubercle at a specific developmental stage, triggering cell death. When researchers blocked Bmp signaling in chicken embryos, the cells survived and the phallus stopped regressing. When they artificially activated Bmp signaling in duck embryos, the duck’s genital tubercle began dying off in the same pattern seen in chickens.¹¹ The finding was striking: the difference between a duck having a penis and a chicken not having one comes down to when and where a single growth-factor signal gets switched on during embryonic development.

This also means that chicken embryos carry all the genetic instructions needed to build a penis. The hardware is still there. It is just that a cell-death signal actively dismantles the structure partway through development. The loss of the bird penis is not about missing genes; it is about a genetic switch that tells existing tissue to self-destruct.

Why Did Natural Selection Favor Losing the Penis

Knowing the developmental mechanism does not explain why losing the penis was advantageous in the first place. Several hypotheses have been proposed, and honestly, none of them is fully settled.

One idea is that females drove the loss through mate choice. In species where females have more control over mating, they may have preferred males with smaller or absent phalluses, perhaps because copulation without intromission gives females more control over which sperm fertilizes their eggs. This hypothesis aligns with what we see in waterfowl, where forced copulations are common and female genital complexity has co-evolved with male phallus size.

Another proposal involves sexually transmitted infections. Birds with intromittent organs have direct tissue-to-tissue contact during mating, which could facilitate pathogen transmission. Losing the organ might reduce that risk. A third idea, proposed by a researcher named Wesołowski, suggested that birds lost the penis to shorten copulation duration and reduce predation risk. But when that hypothesis was tested against data on copulation duration in 243 bird species, copulation did not take significantly less time in species without a penis compared to those with one, effectively ruling out the “faster mating” explanation.¹²

The honest summary is that the female-choice and disease-avoidance hypotheses remain plausible but not conclusively proven. What is clear is that the loss happened multiple times independently in different bird lineages, which suggests it was strongly favored by selection rather than being a random fluke. At least five separate reductions or complete losses have been documented across the bird family tree.¹

The Ostrich Penis and How It Differs From a Duck’s

Ostriches are the largest living birds and possess a sizable phallus, but its structure differs from a duck’s in several ways. Detailed anatomical study of the ostrich phallus found that it is covered by a non-keratinized layered epithelium, meaning its surface is relatively soft and unarmored. It has a sperm groove rather than a closed tube for transporting sperm. It lacks glands along its length, and unlike some other bird penises, it has no elastic core.¹³

The blood vessel architecture in the ostrich phallus is also distinctive. The spongy tissue at the base of the organ contains many small blood vessels, mostly venules, with unusual features: the cells lining these vessels have gaps and openings, and they appear to actively engulf material from the vessel interior.¹⁴ The erectile tissue lining the sperm groove has fewer vessels overall but with even more complex and irregular cell linings. Like the duck penis, the ostrich penis uses lymphatic fluid rather than blood to achieve erection, but the underlying vascular architecture shows how much structural variation exists even among the birds that retained this organ.

A Penis That Grows and Shrinks With the Seasons

In ducks, the penis is not a fixed structure. It changes dramatically with the seasons. The organ grows during late winter and early spring as breeding season approaches, driven by rising levels of testosterone and dihydrotestosterone. After breeding season ends in late summer, the penis shrinks substantially.¹⁵

This was confirmed experimentally in mallards decades ago. Castrating male mallards early in the spring prevented the normal seasonal penis enlargement, while administering testosterone to castrated males could restore growth.¹⁶ The seasonal growth-and-regression cycle means that a duck’s penis measured in July might be a fraction of its March size. For researchers studying these organs, timing matters enormously, and early collectors who examined specimens outside breeding season may have underestimated their true dimensions.

Seasonal regression also means that maintaining a large penis year-round carries costs that natural selection has evidently not favored, even in species that rely on the organ for mating. Growing the tissue fresh each year, rather than keeping it ready permanently, presumably saves energy and may reduce infection risk during the months when it serves no reproductive function. The duck penis, for all its dramatic mechanics, is essentially a seasonal organ, built for a purpose and then partially dismantled until next year.

What About Penile Spines and Surface Features

Some waterfowl penises bear surface structures that add another layer to the sexual conflict story. Certain duck species have ridges, grooves, or small spine-like projections along the shaft. These features vary between species and are thought to play a role in anchoring during copulation or in stimulating the female’s reproductive tract to trigger physiological responses that affect fertilization.

The flexible, non-rigid nature of the everted duck penis, combined with these surface features and its spiral shape, makes it a remarkably specialized tool shaped by millions of years of sexual selection on both sides. The combination of explosive eversion speed, spiral geometry, surface texture, and the ability to navigate complex female anatomy represents one of the more extreme examples of genital co-evolution in the animal kingdom. For an organ that most birds have abandoned entirely, the waterfowl version is an astonishing piece of biological engineering. It underscores a broader pattern in reproductive biology: the traits shaped by sexual selection tend to be the most diverse and most rapidly evolving features in any lineage, even when closely related species have dispensed with the structure altogether.