Stumble upon a moment of sexual intimacy between a pair of wood pigeons in a tree or gulls atop a lamp post, and you might get the impression that birds mate in pretty much the same way that mammals do. But there are some major differences that go beyond the fact that it is feathers that are ruffled rather than fur.
How do birds mate?
The reproductive anatomy of birds is quite different from that of mammals. For a start, it’s not easy to tell from the outside which parts of their anatomy serve a reproductive function. That’s because birds have stuck with a system they have inherited from their reptilian ancestors, in which faeces, urine, eggs and sperm are all ejected through a single orifice, called the cloaca or vent.
In some birds, the cloaca has other additional functions. Over-heated Inca doves, for example, can cool themselves down by exposing the moist lining of their cloaca to the air to encourage evaporation. And among reptiles, spiny softshell turtles can stay submerged in water for long periods by sucking water in and out of their cloaca and extracting the dissolved oxygen.
Do birds have penises?
Another striking characteristic of birds – 97 per cent of species, at least – is that the males lack something possessed by almost all of their mammalian counterparts: a penis.
This inevitably means that sperm cannot be deposited within the body of a female. Instead, when a male bird mounts a female, sperm is transferred by what is known as a ‘cloacal kiss’, which may last only a few seconds. The female then draws the sperm up into her reproductive tract to fertilise her eggs.
The three per cent of bird species that do possess a penis – or more strictly, a phallus, because it evolved independently from the mammalian penis – includes most ducks, geese and ratites (ostriches, emus and relatives).
The organ is stored internally within the cloaca when not in use and is erected by hydraulic pressure. It is not involved in urination, and is used only to deposit sperm closer to the site of fertilisation within the female’s body. Grooves along its length channel the sperm from the cloaca to the tip.
The phallus of the Argentine lake duck averages 20cm in length, about half the length of the bird itself, and may be the longest of any vertebrate, relative to body size.
Phalluses tend to be present in species in which there is intense competition between males over access to females. Male ducks are highly sexually aggressive and routinely attempt to penetrate unwilling females with their corkscrew-shaped phalluses. A female duck’s vagina is also corkscrew-shaped, but coils in the opposite direction to the phallus.
This makes copulation without female cooperation far from easy, and is thought to be the result of an evolutionary arms race between aggressive males and females trying to exert control over who fathers their offspring.
Did you know there are a number of bird species that mate for life?
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Main image: A pair of Cormorants seen sitting on a tree in Assam, India. © Getty Images