What Birds Have a Penis? And Why Most Do Not

Roughly 97% of bird species lack a penis entirely, making birds the only group of vertebrates that reproduce through internal fertilization yet overwhelmingly get by without an intromittent organ.¹ The birds that do have one belong to a handful of ancient lineages: ostriches and their relatives, ducks and geese, and a scattering of other groups. The question of why the penis vanished from so many bird lineages turns out to be more interesting than a simple story about flight, and it involves an evolutionary arms race, a surprising developmental trick, and some of the strangest reproductive anatomy in the animal kingdom.

Which Birds Still Have a Penis

The short list of penis-bearing birds reads like a roster of the oldest branches on the avian family tree. Ratites, the large flightless birds like ostriches, emus, rheas, and cassowaries, all possess a well-developed intromittent phallus. The ostrich penis can measure around 20 cm when flaccid and is bright red.² Waterfowl, the ducks, geese, and swans, are the most species-rich group that retains a penis, with organ lengths that vary enormously across species. Screamers, the chunky South American birds related to waterfowl, also have one. So do cracids, the family that includes guans and curassows. The chachalaca, a cracid, has a well-formed intromittent phallus with a blind tubular cavity and two distinct structural regions.³

Tinamous, ground-dwelling birds from Central and South America that superficially resemble quail, present an in-between case. Some tinamous retain a phallus, but in at least one lineage it has shrunk to a non-intromittent structure, meaning it can no longer be inserted into the female during mating.⁴ A similar pattern appears in the megapodes, the mound-building birds of Australasia. The Australian brush-turkey retains a non-intromittent phallus, while the malleefowl has lost its phallus completely. These cases are important because they show that penis loss did not happen in one grand sweep across birds. It occurred independently in multiple lineages at different times and to different degrees.

Everything else, the vast majority of the roughly 10,000 living bird species, belongs to a group called Neoaves, which includes songbirds, raptors, parrots, hummingbirds, pigeons, woodpeckers, and virtually every bird most people encounter daily. The ancestor of all Neoaves lost the phallus entirely, and no lineage within the group ever re-evolved one.¹

The Cloacal Kiss and How Most Birds Mate

Without a penis, most birds mate through what ornithologists call a “cloacal kiss.” Both males and females have a cloaca, a single opening that serves the urinary, digestive, and reproductive tracts. During mating, the male typically mounts the female’s back, she shifts her tail feathers aside, and both birds press their cloacas together for a brief moment, sometimes lasting only a second or two. Sperm transfers directly from male to female through that fleeting contact.

It sounds precarious, and it is. Sperm transfer during a cloacal kiss is less efficient than through an intromittent organ, which is part of what makes the evolutionary loss so puzzling. You would expect natural selection to favor reliable sperm delivery. Yet the cloacal kiss works well enough for robins, eagles, crows, and the overwhelming majority of bird species to reproduce successfully generation after generation.

Some penis-less males have evolved modest anatomical aids to improve contact. Male songbirds in the fairy-wren family, for instance, develop a prominent cloacal tip, a small muscular appendage at the end of the cloacal protuberance. This tip is not a penis and cannot penetrate the female, but it may help guide sperm transfer. Researchers found that the length of this cloacal tip correlates with testes size across fairy-wren species, hinting that it plays a role in sperm competition.⁵

How a Bird Embryo Grows and Then Destroys Its Own Penis

One of the most striking discoveries in this area came from developmental biology. Chicken embryos actually begin growing a genital tubercle, the precursor structure that would become a penis. The initial signals that kick off this growth are the same ones found in ducks, emus, and even alligators. But partway through development, something different happens in the chicken: a protein called Bmp4 switches on in the tip of the growing tubercle and triggers a wave of programmed cell death. The cells at the end of the proto-penis simply self-destruct, and the structure shrinks away before hatching.⁶

Researchers confirmed this was the key mechanism through a pair of elegant experiments. When they blocked the Bmp signaling in chicken embryo genitalia, the cells survived and the penis kept growing instead of regressing. When they artificially activated Bmp signaling in duck embryo genitalia, the duck’s genital tubercle underwent the same pattern of cell death seen in chickens.¹ In other words, Bmp4 activity at the right place and time is both necessary and sufficient to make a bird penis disappear during development.

Crucially, when the researchers looked at emu and alligator embryos, both of which develop fully functional penises, the Bmp4 expression pattern looked virtually identical to ducks, not chickens.⁷ That finding tells us the Bmp4-driven cell death is a derived trait, something chickens evolved, rather than something ducks lost. The ancestral condition for birds was to have a penis. What changed was the evolution of a developmental off-switch in certain lineages.

Why Did Most Birds Lose the Penis

This is where the evidence gets thin and the hypotheses stack up. Several ideas have been proposed, and none of them has won universal support.

Weight reduction for flight: A penis and the associated vascular machinery add weight. Ditching them could improve flight performance. This is the explanation people tend to guess first, but it has problems. Ostriches and emus are flightless, so they had no flight pressure, yet plenty of flying waterfowl retain large penises. And many penis-bearing ducks are strong, agile fliers.
Reduced disease transmission: An intromittent organ that physically enters the female’s reproductive tract could facilitate the spread of sexually transmitted pathogens. Losing the penis might limit pathogen transfer. Experiments on kittiwakes have demonstrated that bacteria are indeed transmitted during copulation and alter the microbial communities in the female’s cloaca.⁸ But a brief cloacal kiss still transfers microbes, so it is not clear how much protection penis loss actually provides.
Female choice: If females benefit from controlling which male fathers their offspring, they might prefer males with reduced genitalia who are less able to coerce mating. Over generations, this sexual selection could shrink and eventually eliminate the penis.
Faster copulation: Without a penis to insert, mating becomes quicker, which could be advantageous when predators are nearby or when pairs need to copulate frequently in a short breeding window.

Preliminary examinations have not yielded strong support for any single one of these hypotheses.⁹ The real answer is probably some combination, and it may differ across the lineages that lost the penis independently. One recurring theme, though, is the idea that female interests played a major role. In groups where the penis persisted, sexual conflict between males and females seems to have intensified rather than resolved, a pattern that shows up vividly in ducks.

The Waterfowl Arms Race

Duck penises are the most dramatic example of genital evolution in birds. While most duck species have penises in the range of 5 to 9 cm, the Argentine lake duck holds the record with a corkscrew-shaped organ that can reach about 20 cm, roughly the length of the bird’s own body.² The organ is covered in ridges and spines and coils into a counterclockwise spiral. It everts explosively during mating, a process that takes less than half a second.

What makes this even more remarkable is the female side of the equation. In species with long, elaborate male penises, females have evolved correspondingly complex vaginal anatomy: dead-end pouches and spirals that coil in the opposite direction from the male’s organ. A comparative study of 16 waterfowl species found that species with a small, simple penis had short, simple vaginas, while species with long, elaborate penises had longer vaginas with more pouches and tighter spirals.¹⁰ The anatomy of these complex vaginas appears to function as a set of barriers to exclude the male phallus during unwanted matings.

This coevolution is driven by sexual conflict. Forced copulations are common in many duck species, and penis length across duck species is positively correlated with the frequency of these forced matings.¹¹ Males evolved longer, more elaborate penises to succeed at forced mating; females evolved more complex vaginal anatomy to maintain control over paternity. The result is an escalating genital arms race that has produced some of the most extreme reproductive anatomy in the animal kingdom. Research on the explosive eversion of duck penises supports the conclusion that this vaginal complexity functions specifically to exclude the penis during forced copulations.¹²

This arms race offers a window into why penis loss in other bird lineages might have been driven partly by female preferences. In species where forced copulation was less of a problem, or where females could already exercise mate choice through other behavioral mechanisms, there may have been no selective pressure to maintain an elaborate penis, and every reason for females to prefer males without one.

A Different Kind of Erection

Bird penises work differently from those of mammals and reptiles at a basic mechanical level. In most vertebrates, erection is driven by blood filling vascular chambers. In birds, a major evolutionary transition occurred: the erection mechanism switched from blood-based to lymphatic.¹³ Lymph, a clear fluid that circulates through the body’s lymphatic system, fills the tissues of the bird penis to produce tumescence.

The ostrich penis, for example, is a sophisticated fibro-cavernous structure with distinct regional differences along its length. The front and middle portions contain dense connective tissue and are packed with specialized touch receptors called Herbst’s corpuscles. The back portion has broader lymphatic channels for fluid exchange. The base is anchored by layers of skeletal muscle.¹⁴ Unlike a mammalian penis, which typically has a urethra running through it, the ostrich penis has an ejaculatory groove along its ventral surface. Semen runs along this groove rather than through an internal tube.

The tinamou penis offers another variation on the lymphatic theme. When erect, the eversible portion of the organ evaginates, turning inside out as it emerges from the cloaca, and takes on a spiral shape directed toward the left. The walls of the organ contain a wide lymphatic space that encircles the entire structure, and the vascular bodies at the base form a continuous system involved in generating the erection.¹⁵ This lymphatic erection system is found across all the bird lineages that retain a penis, suggesting it evolved early in avian history, before the penis was lost in most groups.

The Cloacal Protuberance in Songbirds

Even though songbirds and other Neoaves completely lack a penis, their reproductive anatomy is not entirely featureless during breeding season. Male songbirds develop a cloacal protuberance, a visible swelling around the cloaca that stores sperm and helps with transfer during the cloacal kiss. This structure changes size dramatically with the seasons. In blue tits, the cloacal protuberance shrank by more than half in actively copulating males between the breeding season and the post-breeding period.¹⁶

The cloacal protuberance is sometimes used by field biologists as a quick way to assess whether a male bird is in breeding condition. A large, swollen protuberance indicates the bird is reproductively active, while a small one suggests the breeding season has ended or not yet begun. This seasonal plasticity underscores how much energy birds invest in reproduction only when they need to, a pattern that contrasts with the permanent anatomical commitment of maintaining a penis year-round.

Penis Plasticity in Ducks

Even among the birds that do have a penis, the organ is not necessarily a fixed structure. Research on waterfowl has found evidence of phenotypic plasticity in penis size, meaning individual males can grow longer or shorter penises depending on their social environment. Male ducks housed with more competitors developed differently than those in less competitive settings, and some species showed delayed reproductive maturation when competition was high.¹¹

This plasticity makes biological sense. Growing and maintaining a long, elaborate penis is metabolically expensive. If a male finds himself in a social group where his chances of mating are low, investing less in genital tissue and more in body condition or survival could be a better strategy. Conversely, in a highly competitive environment with many rivals, a longer penis might pay off by improving mating success. The fact that the organ can respond to social cues adds another dimension to the already complex picture of avian genital evolution. It suggests that even in the lineages that kept the penis, the structure remains under intense and ongoing selection pressure rather than being a stable anatomical fixture.

Multiple Losses, Not One

One common misconception is that birds lost the penis in a single evolutionary event, perhaps tied to the origin of flight. The evidence points to something messier. The phallus was lost or reduced independently in at least five separate lineages: once in the ancestor of all Neoaves, at least once within the galliforms (chickens and their relatives), and separately within tinamous and within megapodes.⁴ Each loss appears to have occurred through its own evolutionary pathway, though the Bmp4-driven developmental mechanism discovered in chickens may turn out to be a shared tool that different lineages independently co-opted.

The fact that the phallus is considered the ancestral condition in birds, inherited from their dinosaur ancestors and retained in the most ancient living bird lineages, reinforces this picture.³ The penis did not evolve freshly in ducks and ostriches. It was always there, and most birds got rid of it. Crocodilians, the closest living relatives of birds, have blood-vascular penises, and the shared ancestor of birds and crocodilians almost certainly did too. What happened in birds was not the invention of a new trait in a few lineages but the repeated deletion of an old one across many.

1
Current Biology. Developmental Basis of Phallus Reduction in Birds
2
Oxford Academic (The Auk). The 20-cm Spiny Penis of the Argentine Lake Duck (Oxyura vittata)
3
Acta Zoologica. Morphology of the copulatory organ in Ortalis canicollis (Aves: Cracidae) and early evolution of the phallus in birds
4
Journal of Avian Biology. Independent evolutionary reductions of the phallus in basal birds
5
Journal of Avian Biology. Good vibrations? Structure and function of the cloacal tip of male Australian Maluridae
6
PubMed. Developmental basis of phallus reduction during bird evolution
7
Current Biology. Genital Evolution: Cock-a-Doodle-Don’t
8
Europe PMC. Sexually transmitted bacteria affect female cloacal assemblages in a wild bird
9
Current Biology. Cock-a-Doodle-Don’t
10
PubMed Central. Coevolution of Male and Female Genital Morphology in Waterfowl
11
BioOne Complete. Evidence of phenotypic plasticity of penis morphology and delayed reproductive maturation in response to male competition in waterfowl
12
PubMed Central. Explosive eversion and functional morphology of the duck penis supports sexual conflict in waterfowl genitalia
13
Journal of Zoology. The erection mechanism of the ratite penis
14
Journal of Experimental Zoology India. Histological architecture of the male copulatory organ in a common ostrich (Struthio camelus)
15
PubMed. Morphology of the copulatory apparatus of the spotted tinamou Nothura maculosa (Aves: Tinamiformes)
16
Ardea. Volume of the Cloacal Protuberance as an Indication of Reproductive State in Male Blue Tits Cyanistes caeruleus