Spring is the breeding season for most birds, but how do birds mate? Coming together in sexual copulation is essential to fertilize eggs to raise young birds, but the sex act is only a brief part of the courtship and pair bonds between birds.

The Reproductive Anatomy of Birds

Most birds do not have the same reproductive body parts as mammals. Instead, both male and female birds have a cloaca. This opening (also called the vent) serves as the bodily exit for their digestive, urinary, and reproductive systems. This means the same opening that excretes feces and urine is the opening from which eggs are laid. During the breeding season, the cloaca swells and may protrude slightly outside the body, while during the rest of the year it is much less prominent and not typically visible.

When birds are ready to breed, their reproductive organs, the testes and ovaries, swell and produce the sperm and ova. Male birds store sperm in their cloaca until an opportunity to mate arises, and females will receive that sperm into their cloaca before it travels deeper into their bodies to fertilize their ova and begin egg formation.

1:10 Watch Now: How Do Birds Mate?

Bird Courtship

The courtship between a pair of birds can last much longer than the actual act of copulation. Courtship behavior may include several stages, from initially claiming territory to actually wooing a prospective mate with visual and auditory displays such as stunning plumage, spectacular flights, intricate songs, or even elaborate dances. The courtship period is when a male bird shows off his health and strength to convince a female that he is her best possible mate and will help her create the strongest, healthiest chicks with the best chance to survive.

Illustration: The Spruce / Kaley McKean

How Birds Have Sex

Once a female bird is receptive to a mate, whether it is a new mate every breeding season or simply renewing ties with a life-long partner, the actual mating can take place. The positions and postures birds assume to mate can vary, but the most common is for the male bird to balance on top of the female. The female may hunch, lay down, or bow to give the male easier balance, and both birds face the same direction. She will then move her tail aside to expose her cloaca to his reach, and he will arch or curl his body so his cloaca can touch hers. The brief rubbing of cloacas may last less than a second, but the sperm is transferred quickly during this "cloacal kiss" and the mating is complete. The balancing may take longer as the birds stay touching one another, and several "kisses" might occur within a few moments. Birds will remain excited by their hormones for a week or more and may mate several times during that period to increase the chances of successful insemination.

Some bird species, most notably several species of swans, geese, and ducks, do not have cloacas, but instead male birds have an actual phallus (penis) that is inserted into the female during mating. The penis is formed by an extension of the cloacal wall, and unlike mammals, is erected by lymph rather than blood. Having a penis helps different types of waterfowl mate in the water without the sperm washing away from an exposed cloaca. Several other bird species, including cassowaries, kiwis, and ostriches, also have penises rather than cloacas, but the mating act is still only a brief encounter.

After mating, the sperm travels to the ova for fertilization. Eggs may be laid in just a few days or it may be several months before eggs are ready to be laid and the final brooding of the nest begins.

If You See Mating Birds

Many birders are at first thrilled to see unique bird behavior, then quickly become embarrassed or uncomfortable when they realize they are watching birds have sex. Because the mating act is so brief, being observed does not typically disturb the birds, but it is important to realize that this is still a delicate time for bird pairs.

If you see mating birds, it is best to keep your distance, as approaching more closely may spook the birds and force them to leave, which can interrupt their courtship or hurt their pair bond. This may also cause difficulties for raising a brood or completing a successful mating if the pair splits prematurely. If they are severely disturbed, the birds may leave their carefully chosen territory to relocate to a less suitable area that may not provide for all their hatchlings' food, shelter, and other survival needs.

After the birds have mated, they may remain nearby to nest and raise their brood. This can provide a unique opportunity for birders to observe a growing bird family, but the same caution should be taken to stay away from the nest in order to safeguard young birds. Too much attention can distress parent birds, forcing them to abandon the nest or hatchlings. Drawing attention to the nest can also attract predators, and birders should take great care to not disturb nesting birds in any way.

Seeing mating birds can be exciting, and it's a great reminder of how special spring birding can be. By understanding how birds reproduce, birders can better realize what unique behaviors they see in the field and can take steps to protect nesting birds and their young.