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Birds are well-known for their incredible and ornate mating displays and rituals, ranging from the careful planning and insane plumage of the male Lyrebird to the aerobatic feats of various male hummingbirds and the bizarre rituals of various Birds-of-paradise. Whilst much discussion around bird reproduction revolves around mating displays, it’s also important to consider the question of how do birds actually mate?
The vast majority of birds mate only fleetingly, the entire act lasting just seconds, even when the mating display itself is long-winded, carefully planned, and the result of potentially millions of years of evolution!
Rather than sex, the act of birds mating is usually called a cloacal kiss - the cloaca is an opening that some 97% of male and female birds possess. The cloaca itself does not mirror a penis in any way - it is an opening that is connected to the reproductive organs. The remaining 3% or so of birds do possess some sort of appendage that could be classed as a penis, though it is still far removed from the mammalian penis.
Avian mating rituals inspired many of Darwin’s early theories of evolution, where he hypothesised that competitive mating displays helped the female birds select for the fittest mates whilst also helping birds discern between different species.
Not only do mating displays tickle the senses, attracting females to those males that go the extra mile to impress, but they also secure the fitness of future generations who will also then become increasingly adept at the art of reproduction.
Read on to explore the nuances of bird mating and its associated behaviours!
A pair of Red-crowned Cranes during courtship
The cloaca, which means “sewer” in Latin, is a single opening for the intestinal, genital and urinary tracts. Not only is the cloaca used in copulation and mating, but it’s also where a bird releases its urine and faeces. It usually takes the form of a small hole near the back of the bird that is obscured by feathers.
It’s not just birds that have a cloaca, but amphibians, reptiles, birds and elasmobranch fishes (e.g. sharks).
Whilst avian mating displays are often quite complex and incredible, the actual act of sex is usually much more utilitarian. In other words, sex in birds really just serves a purpose and doesn’t elaborate much beyond the fertilisation of eggs, which is, after all, the ultimate goal of procreation.
In contrast to having two anatomically different organs, i.e. the penis and vagina in mammals, the vast majority of birds have a cloaca. The cloaca is just an opening that connects to a bird’s reproductive and digestive organs.
On the outside, the male and female cloacas look more-or-less the same. During the breeding season, the cloaca becomes swollen and after a successful mating or courtship ritual, the male proceeds to rub his swollen cloaca onto the female’s cloaca. Usually, the male will hop onto the female’s arched back who will move her feathers to allow the male’s cloaca to align in close proximity, though the organs themselves may never touch.
Sperm from the male enters the female and travels towards the egg, which is hopefully successfully fertilised.
A pair of Kingfishers mating
As always, there are exceptions to the rule. Ducks, swans, geese and many flightless birds like ostriches and emus have penises, as do Tinamidae, a family of shy ground-dwelling birds.
Common estimates suggest that around 3% of all birds have some form of penis or penis-like appendage, whereas the remaining 97% possess only cloacas.
Birds that do possess penises do use them for penetrative sex, but there are still crucial differences between bird penises and those of mammals and other animals. Perhaps the most strange difference is that bird penises become erect via lymphatic fluid rather than blood. Lymphatic fluid is under much lower pressure than blood, so most bird penises only remain erect for a matter of a few seconds at the very most.
Members of the Tinamidae family have penises
It’s an excellent question that scientists struggle to answer to this day.
Studies of various bird embryos revealed the genital tubercle, the precursor of the penis, or phallus, does indeed grow in bird embryos to some extent, but its growth is stunted.
Specifically, the development of the penis is stunted by a gene called, Bmp4, which is involved in the development of avian bone, muscle, cartilage, bills and limbs. Other Bmp proteins led birds to lose their teeth, amongst other avian-specific evolutionary developments. Bmp4 signals a process of apoptosis, which essentially means the cells associated with penis development self-destruct. The cells present for the growth of a penis die in most species of birds, hence why they have cloacas instead.
The corkscrew penis of a Mallard duck
There is certainly no confident theory as to why this process evolved, and why a minority of birds retain their penises. Given that most birds that retain their penises live primarily on the ground or in water, some argue that penises assist in the process of copulating underwater.
Similarly, penises may be cumbersome in-flight, but some ducks, swans and geese are great migrators - so that doesn’t make much sense.
The process of penetrative sex may also leave birds vulnerable, or perhaps females select for males with small penises. And then, of course, it might simply be that penises are surplus to requirements in birds - perhaps they make up for what they lose in sex by the ornate nature of their complex mating displays. In some birds, such as the ostrich, penises might still be essential for mating rituals and competition between males.
Birds use a tremendous range of techniques to attract mates. There are 3 main components to these rituals or displays:
Almost all male birds sing or use vocalisations to try and attract a mate. Singing announces the presence of a male but is also a competitive event where males try and outdo each other’s songs. Some birds even try to cut each other off mid-song to interject with their own competitions. These competitive behaviours likely go some way to explain why bird song is so complex.
The courtship display of a Frigate bird
You don’t need to dig much deeper to find some truly bizarre and fantastic avian mating displays.
Male Birds-of-paradise from Papua New Guinea exhibit some of the strangest, most variable and colourful plumage and patterns of any bird, dancing and prancing around the female to woo her.
Falcons fly acrobatically through the sky in a display of daring midair pursuits; diving, climbing, twisting and turning at high speeds to impress a potential partner. Hummingbirds do the same, darting around at breakneck speeds whilst flying extremely close to nearby terrain to show off their machoness. The Lyrebird has a very complex mating display that involves both song and dance on a carefully prepared mound, as well as a surreal display of its incredible plumage.
And then, let’s not forget the Peacock, which has evolved truly outlandish (and rather unwieldy) plumage almost solely for the purposes of attracting a mate.
Perhaps one of the most surreal - or almost mystical - mating displays is that of the Bowerbird. The Bowerbird is a skilful architect that creates a complex structure for its mate, which is decorated with carefully chosen stones, petals, trinkets and even plastic.
Bowerbird collecting blue objects to attract a mate
Birds do not get pregnant in a mammalian sense. In the breeding season, where the female is ready to lay eggs, she will need a male to fertilise those eggs. Once fertilised, an egg might be laid within 24 to 48 hours on average. At this stage, whilst the egg has been fertilised, very little growth has taken place inside the egg.
The embryo only begins to develop once the egg has been laid, during the incubation period. Throughout the incubation period, the fertilised embryo will develop quickly, essentially growing from an array of cells suspended in a liquid into a baby bird, or chick, that hatches at the end of the incubation period.
A pregnant American Robin (Turdus migratorius), stood nearby to her nest
Birds mate over the course of just seconds, so you’re more likely to see the courtship ritual that leads to mating rather than the act of mating itself!
In general, though, birds mate in much the same way as other animals. The female bends over, arching her back, and the male presses himself against her from behind. The difference is, in birds, there is usually no actual penetration - copulation occurs by the organs of the male and female rubbing together fleetingly.
A pair of House Sparrows mating
Birds can lay eggs without mating, though this is much more common in domesticated birds (e.g. chickens) than wild birds. Chickens have been bred to create eggs near-constantly without the presence of a male bird - their hormones trigger them to lay infertile eggs (which is what humans consume as food).
In the wild, birds will not expend the energy required to lay eggs unless there is a male present to fertilise the eggs. This will help trigger the hormonal cascade that instructs female birds to produce eggs. Most female birds will not form eggs unless:
Once these precursors are in place, the female bird’s body will be ready to lay eggs. In some scenarios, a bird’s egg-laying clock will misfire, causing them to lay an infertile egg without ever mating. This can happen with pet birds, like female parakeets. Overall, it’s pretty rare for wild birds to lay eggs without mating.
Chickens are able to produce eggs without mating
All female birds lay eggs, but male birds don’t. By definition, to be a bird, the animal must lay eggs. No birds give birth to young in any other way.
There may be some scenarios where a birth defect means that a bird cannot lay eggs, but this is tangential to the question of whether all bird species lay eggs, to which the answer is an unequivocal yes.
The reasons why some animals have evolved to lay eggs, which then develop and hatch outside of the body, and why some give birth to live young (like humans do), are still debated to this day.
Egg laying presents several advantages in that the eggs can quickly be laid after being fertilised, essentially freeing the female bird from everything but incubation duties until the eggs hatch. This enables the bird to hunt for food, etc, all without carrying the payload of several developing birds inside of her. This also means that birds can have more offspring - in some species, as many as 15 eggs are laid in one clutch.
Also, egg laying enables the male to take on incubation duties, or even allows other birds from within the same brood or family unit to incubate the eggs for a period.
Egg laying provides the flexibility and volume that birds need to raise future generations.
Mallard duck nest, with a large clutch of eggs
The process of birds mating with other species or breeds is called hybridisation. As many as 16% of bird species are known to hybridise. Notable examples include the Swoose - a hybrid between a swan and a goose and the Rackelhuhn, a grouse hybrid, as well as chicken and pheasant hybrids, various parrot hybrids, corvid hybrids and Birds-of-Paradise hybrids.
For hybridisation to be successful, the birds have to be closely related, which is not always the case even within the same family or genus.
A close up profile of a Swoose, or Swan Goose
Some birds mate even when they are not planning to lay eggs - eagles are just one example of birds that tend to do this. Most birds do only mate in the formal breeding season, however.
Conventionally, the breeding season is defined as spring, but it depends on where you are in the world. In much of Europe and North America, the months of March through until June are probably the busiest for breeding birds. Some species will mate much earlier, though - some species of owls tend to lay eggs in the midst of winter, for example.
Closer to the equator, the conventional breeding season tends to become a little more fractured. Since it’s often warm all year round, birds may instead tweak their breeding season to other climatic factors such as rainfall.
The mating ritual of Woodpigeons
Mating may occur all throughout the season, but the laying of eggs and subsequent rearing of chicks typically only occurs once or twice per year.
Many females will attempt to lay a new clutch if the first fails early on in the breeding season, e.g. the nest collapses. But otherwise, most species of birds have just one round of offspring per season.
Other species of birds do not mate every year. Some eagles, like the Crowned Eagle and Harpy Eagle, breed every 2 to 3 years. The Wandering albatross is the same, breeding just once every other year. The chicks of these birds need to be carefully raised for nearly a year, or even longer, meaning that they cannot raise a new brood every year.
Female birds develop eggs in the breeding season, but whilst domesticated birds like chickens are bred to produce egg after egg even where there is no sperm present, wild birds will typically only produce eggs once there is sperm present to fertilise the egg. When laying is imminent, female birds may appear plumper than normal.
Pregnant European Robin
Birds do not go into labor, as birds do not give birth to live young. Female birds lay eggs rather than give birth in a mammalian sense. The egg is just a fertilised embryo at the point it is laid - there is no ‘living’ bird inside of it. The process of egg-laying itself can be sore, though!
Periods, which involve the shedding of the uterus’s endometrial tissue as blood, do not occur in birds. Birds do not have a uterus - they instead have an oviduct and a shell chamber. There is no hormonal process in birds that resembles that of a period.
Male and female Canadian Geese incubating their eggs in the nest
It is possible for either the female to solely incubate (sit on) the eggs, the male to solely incubate the eggs, or for both the male and female to incubate the eggs.
Many species of birds take part in biparental care, meaning both the male and female assist in rearing the chicks. Here, it is common for the male to help incubate chicks. Even the males of bird species that are primarily polygamous, e.g. the ostrich, can help incubate the eggs. In some polyandrous birds, like jacanas and phalaropes, it might be solely the male that incubates the eggs.
No, birds do communicate by rubbing their bills together or ‘kissing’, but usually mate using cloaca, which is an opening connected to their reproductive organs.
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