In this article we explain how birds mate, examining the physical process of reproduction itself, with additional observations on associated displays and rituals.
We humans can easily anthropomorphize the courtship that precedes the congress of birds. The often-spectacular male display is full of bravado and macho "look at me" posturing whilst the female, in contrast, coyly and demurely feigns complete disinterest – the very model of playing hard to get.
The avian act of procreation itself that follows - should the male get his way - is, however, completely alien to us and involves, shall we say, no organs that we would recognise. It is also startlingly brief.
A pair of Red-crowned Cranes during courtship
The males of a small minority of birds such as ducks, swans and geese do have an appendage which is classed as a penis (albeit quite unlike the human one) and the females retain a vagina. However, that minority represents only around some 3% of the thousands of species that exist.
It’s thought that, earlier in their evolution, all species did have them however, as from examinations of embryos at various stages of development, we can see the penis and vagina begin to form in most birds but then shrink or disappear altogether.
Science offers some theories, but no complete explanation, as to why this is. Perhaps a penis is a disadvantage in flight, or that the longer coupling involved in penetrative reproduction left birds vulnerable to predators and so evolution selected it out.
So, without penetration, how do those remaining 97% of species mate?
Males have instead a non-penetrative organ called the cloaca, which replaces the vagina in females. Known too as a vent, this bodily cavity stores the sperm prior to mating. In both genders the cloaca is a part of the digestive tract that ejects excreta, and is also the passage through which eggs pass in the case of the female. Rather unromantically, but perhaps appropriately, the word derives from the latin for “sewer”. Cloacae are to be found in sharks, reptiles, frogs, the platypus and echidna as well.
If the male is able to prove his mettle by - depending on the species - building a nest, singing a serenade, displaying spectacular plumage, showing athletic prowess, cutting some fancy dance steps or resorting to bribery in the form of seeds or sparkly artefacts, the female will signal her readiness to reluctantly succumb to mating by lifting her tail in the air. She will then often lower herself closer to the ground (it's often said that birds mate on the wing but - apart from the swift - it's a fallacy) to help the male balance as he mounts her, lowering his rear as she moves her tail feathers to one side until the cloacae touch for a fleeting moment. This is the “cloacal kiss” – a brief, sub-second, moment of intimacy during which the necessary fluid is transferred.
That one moment is long enough for the sperm to enter the female where, from her own cloaca, they will swim a short distance in order to fertilize her eggs. However, should we be concerned at this apparent lack of opportunity for sensual fulfilment, it’s reassuring to know that many species will repeat this act over and over again. It is, after all, something of a hit or miss procedure.
Given the rather precarious nature of this exchange, the retention of a penis and vagina in waterfowl would, we might conjecture, also make excellent evolutionary sense. Any dilution of the semen in water would plainly lead to a lower percentage of successful fertilizations.
Gestation periods vary widely but, because the chicks will continue to mature in their eggs, some are laid just a few days afterwards, the protective shell hardening in the last few hours before.
A pair of Kingfishers mating
As we touched upon, the prosaic nature of fertilization is in stark contrast to the varied courtship strategies birds demonstrate. Whilst many are astonishingly intricate and involved, we should remember that many birds are monogamous and stay together until death. Their displays, first time around at least, are therefore best viewed as a proposal of marriage rather than a more cursory chat up line for a one-night stand, and so we might feel they are more in proportion to all the effort and drama that is involved.
Those interested in mating displays will find several fascinating clips elsewhere on the web and I recommend the Bowerbird, Peacock, Manakin (and other Birds of Paradise - around 40 varieties), the albatross, grebe and Victoria’s Riflebird as some of the most engaging. For the brave-hearted, there are also many images of the wonderful world of waterfowl penises.