How Do Birds Mate? Most Lost the Penis Their Ancestors Had

If you have spent time around city pigeons, you have probably watched a male puff up his chest and strut in circles around a female, bowing and cooing. He is courting her without a penis.

In 2013 National Geographic listed pigeons among the birds that have lost the organ completely, along with eagles, owls, gulls, hummingbirds and woodpeckers.

How do birds mate?

Most birds mate with a “cloacal kiss.” The male mounts the female and the two press their cloacas together, the single opening birds use for droppings, eggs and sperm, and sperm passes from him to her in a contact that may last only a few seconds. Only about 3% of bird species have a penis that can be inserted.

Those are mainly ducks, geese and swans, plus ostriches, emus and other ratites, many tinamous and the cracids of Central and South America. Chickens and most other land fowl keep a small penis that cannot be inserted, and most other birds have none.

The female takes an active part. When captive female house sparrows initiate mating, they adopt “a crouching position with their wings quivering and their posterior end held upright,” and both birds bend their tails so the cloacas meet, a 2018 study in Scientific Reports describes. Mating comes in bouts of repeated rapid mounts, and not every mount includes cloacal contact.

The sperm still ends up inside the female

Mating through the cloaca, the opening that delivers white urine and dark feces in a single dropping, still ends in internal fertilization. “All birds reproduce by internal fertilization,” a 2013 paper in Current Biology states in its abstract. That sets them apart from eels, which are thought to release eggs and sperm into open water.

Inside a hen, sperm move up through the vagina, which opens into the cloaca. Only about 1% of them enter the sperm storage tubules at the junction of the vagina and the uterus, a 2025 review in The Journal of Poultry Science reports, and sperm can survive there for extended periods. That is why a hen keeps laying fertile eggs long after mating: the reported duration is 2 to 3 weeks in chickens, 10 to 15 weeks in turkeys and 1 to 2 weeks in Japanese quail.

Fertilization happens at the upper end of the oviduct, in the infundibulum, where sperm meet the ovum before its white, membranes and shell are added further along. Laying itself does not depend on mating: grocery store eggs “are from hens raised without roosters,” notes a 2020 review in Sensors, and an unfertilized ovum still travels the oviduct and is covered in egg white like any other.

Only the oldest bird lineages kept a penis

Biologists usually call the bird organ a phallus, because it evolved separately from the mammalian penis, as BBC Wildlife’s Stuart Blackman points out. The species that have one all belong to “basal lineages of extant birds,” Patricia Brennan and colleagues wrote in PLOS ONE in 2007, meaning the early branches of the bird family tree.

Group	What the males have	Examples
Neoaves, the vast majority of species	No penis	Pigeons, owls, eagles, gulls, wrens, hummingbirds
Land fowl (galliforms)	A small penis that cannot be inserted	Chickens, quail, pheasants
Waterfowl (anseriforms)	A penis that can be inserted, spiralled counter-clockwise	Ducks, geese, swans
Large flightless birds (ratites)	A penis that can be inserted	Ostriches, emus
Tinamous	In many species a penis that can be inserted; one that cannot in the genus Crypturellus	Ground birds of Mexico, Central and South America

Exceptions run through the groups, especially the land fowl. The cracids, a family from Central and South America, have penises, while the malleefowl, an Australian mound-builder distantly related to the chicken, has lost its penis entirely.

In ducks, the female’s anatomy has evolved along with the male’s. Across 16 waterfowl species, Brennan’s team found that every male penis spiralled counter-clockwise, and that where males had long penises, females had longer, more elaborate vaginas, with dead-end sacs and coils that turn clockwise, the opposite way.

In a 2010 follow-up, high-speed video caught the Muscovy duck’s 20 cm penis everting in an average of 0.36 seconds. It everted less successfully into glass tubes that coiled clockwise or bent at 135 degrees, shapes that mimicked the female’s, which the authors took as support for the idea that her anatomy helps exclude males during forced matings.

In 2001 Kevin McCracken’s team described in Nature an Argentine lake duck whose corkscrew-shaped penis measured 42.5 cm, as long as the bird’s body, according to ABC Science. The earlier estimate for the species had been 20 cm.

A chicken embryo starts to grow a penis, then its cells die

A chicken embryo starts to grow one too. In a 2013 study led by Ana Herrera, a graduate student working with Martin Cohn at the University of Florida, embryos of a Pekin duck and a domestic chicken began the same way: small swellings fused into a stub called the genital tubercle, which grew over the first week or so, as Ed Yong described the work for National Geographic.

The duck’s kept growing into a coiled penis. The chicken’s stopped around day 9, while it was still small, Yong reported.

The cause was cell death. A gene called Bmp4 switches on at the tip of the tubercle and, in the words of the university’s announcement, “turns on a cell suicide program that causes the tubercle to regress.” In ducks and emus, Bmp4 is not activated there.

The team implanted tiny beads carrying a Bmp-blocking protein into the tubercles of chick embryos, and where the beads sat, the cells survived and growth continued. Bmp protein added to duck tubercles the same way caused cell death.

“It was surprising to learn that outgrowth fails not due to absence of a critical growth factor, but due to presence of a cell death factor,” Cohn told National Geographic.

The result is small: Yong described chickens’ penises as “tiny nubs that are no good for penetrating anything,” and a 2015 paper from Herrera, Brennan and Cohn calls the chicken’s a “non-intromittent penis.”

The gene result covers chickens, not every bird

The University of Florida announced the paper under the headline “Activation of cell death gene triggered loss of phallus in bird evolution,” which reads as if one gene explained the whole class. The paper is narrower.

It compared land fowl with waterfowl, using emus and alligators as outgroups, and its abstract concludes that Bmp4-driven cell death explains the reduction “in galliform birds,” the order that includes chickens and quail. Yong noted the limit at the time: Cohn’s team had “only compared two groups.”

Neoaves, which Yong describes as including “the vast majority of bird species,” sits on another branch entirely, and none of its members has a penis. A 2008 survey in the Journal of Avian Biology by Brennan and colleagues counted two reductions already known, the land fowl’s switch to a penis that cannot be inserted and “a complete loss of phallic structures in the ancestor of Neoaves,” and reported three more: penises that cannot be inserted in a genus of tinamous and in the Australian brush-turkey, and a complete loss in the malleefowl.

Why birds lost the organ is still uncertain, the Florida release says. For land fowl, it set out two of the hypotheses Herrera described: a side effect of changes in Bmp genes, which also help shape beaks, feathers and limbs, or female choice for males with smaller genitalia. “The females want to have more control over copulation, reproduction and mate choice,” she said.

Most matings take seconds, but not the aquatic warbler’s

In most birds the contact is brief: Blackman writes that the cloacal kiss “may last only a few seconds.” The aquatic warbler, a small songbird that breeds in sedge marshes in eastern Europe, takes far longer. In a 1995 study in Ibis, Schulze-Hagen and colleagues timed mounts in hand-reared, captive birds at an average of 23.7 minutes, with about six cloacal contacts in each copulation and the male staying on top of or directly behind the female in between.

The authors suggested the long mount may be “contact mate guarding,” which they called “a behaviour not previously recorded in birds.”

The pigeon bowing on the pavement and the rooster in the farmyard mate the same way, though they got there by different routes: the pigeon’s ancestors lost the penis outright, while the rooster’s embryo starts to grow one until Bmp4 sets its cells dying. Strictly FYI.