One of the nicest terms in zoology, a cloacal kiss is what happens when a mummy bird and a daddy bird love each other very much. They bring their all-purpose excretory and reproductive openings (called cloacas) together, and sperm is transferred from the male to the female. Some romantic zoologist considered this contact not dissimilar from two mouths kissing, and the name has stuck.
What follows is the full story of the fantastic journey of bird sperm. In human relations, romance typically happens by moonlight; however, in the animal kingdom the light of the sun is more important. Like humans marking off the passing year by days getting shorter and longer, birds too can judge the approach of breeding season by the changing light. Surprisingly, this is not done with their eyes; the sun's rays shining through the bird's skull stimulates photoreceptors in the brain and causes the production of sex hormones and other effects.
In males, as well as causing changes in plumage and behaviour, this awakens the testes, which swell up to become hundreds of times bigger than normal and often change colour. The enlarged testes can make up a tenth of the bird's body weight; the left is generally larger than the right. Sperm is made in the testes and passes via the vas deferens. As with mammalian sperm, each spermatozoon has a head and a tail as well as hormones to allow it to penetrate the egg. Its journey down can take days, and (depending on species) it can be stored for additional time either in the vas deferens or in the seminal vesicle, a convoluted structure at the end of the vas deferens. This opens into the urodeum, the part of the cloaca where the ureter also empties uric acid from the kidneys.
As mentioned, birds have a single reproductive and excretory opening, the cloaca. Combining the functions of mammalian genitals and anus, this excretes feces and nitrogenous waste (as uric acic), as well as emitting sperm in the male and receiving sperm and laying eggs in the female. In a few species the males have penises: anseriformes (waterfowl), ratites (ostriches etc), tinamous, and storks, which protrude from the cloaca for intercourse but are normally stored inside; however most birds have no such organ. For reproduction to occur in those species without penises, the outer edges of the cloaca of the male and female must be brought together to allow the transfer of sperm. This touching of cloacas is called the "cloacal kiss".
Bird mating rituals are complex and highly varied, but (as with human courtship) they all end with the same thing. For intercourse to occur, the female adopts a mating position, which signals to the male that she is sexually receptive. This position varies between birds but generally includes adopting a submissive posture and moving the tail upwards and/or to one side. The male typically approaches diagonally from behind her and climbs onto her back or simply stands very close to her, getting into position to allow the tips of their cloacas to touch. Normally the anterior of the male's cloaca lines up with the posterior of the female's, and vice versa, i.e. the two birds' cloacas are inverted relative to each other.
The transfer of sperm can take less than a second, although some birds linger for up to an hour. In most species, copulation takes place on land, although a few (e.g. white-throated swifts) mate in the air, and many water birds copulate while afloat. (It would probably be simpler from the point of view of the avian kama sutra if the cloacas were on the front, but that's nature for you.)
Inside the female, sperm is stored in the cloaca at the lower end of the oviduct (egg tube); it can be kept for many days, up to two months in the case of turkeys. This means that copulation does not need to occur frequently, although in many birds it does, owing to sperm competition. Each sperm carries half of the chromosomes needed for making a bird cell, the remainder coming from the female within an ovum, or egg (the exact number of chromosomes varies between species or genera). Sex chromosome arrangement in birds differs from in humans - male birds have two Z chromosomes while females have a Z and a W - however the principles are similar.
Females in most species have a single ovary, the left, though some e.g. Falconidae (falcons) have two. The ovary is connected to the cloaca via a long tube, the oviduct, which has many parts with different functions. On leaving the ovary, the egg is already surrounded by layers of yolk; it passes into the ostium, the first stage of its journey through the oviduct. Next is the magnum, where the egg is fertilized by a sperm cell which has swam up the female's oviduct; the sperm fuses with the nucleus of the egg and they create a cell that is ready to divide. Albumen (egg white) is added to the fertilized egg in the magnum; then the shell membranes are lain down in the isthmus.
The shell hardens in the uterine area, the final part of the oviduct, and the egg is stored at the base of the oviduct until it is expelled from the cloaca by the vaginal muscles. Then out it pops, and it is either eaten or broken, or it hatches into a new bird.
A cloacal kiss also occurs in a few lizards, such as tuataras, which have no male external sexual organ (most other reptiles have one or even two penises, housed in pouches off the cloaca).
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Paul R. Ehrlich, David S. Dobkin, and Darryl Wheye,
"Copulation",
Stanford Alumni website,
1988,
http://www.stanfordalumni.org/birdsite/text/essays/Copulation.html
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D. Hansen,
"Courtship and Mating",
Hamilton and District Budgerigar and Cage Bird Society website,
http://home.ca.inter.net/~dhansen/court.html
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Bruce Musico,
"Sphenodon punctatus (tuatara)",
Animal Diversity Web,
http://animaldiversity.ummz.umich.edu/site/accounts/information/Sphenodon_punctatus.html
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Darrel K. Styles,
"A brief review of avian genetics",
Old World Aviaries website,
http://www.oldworldaviaries.com/text/styles/genetics.html
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David Swanson,
Ornithology lecture notes,
University of South Dakota website,
http://www.usd.edu/biol/faculty/swanson/ornith/lec10.html
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"The Sex Lives of Birds: The Honeymoon",
Ed. Christine Tarski,
Birding section at About.com,
http://birding.about.com/library/weekly/aa071700c.htm