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Owls are generally monogamous, mating as an exclusive pair either for the season, such as the short-eared owl, or for life, such as the tawny owl. The courtship rituals of the owl varies from species to species, but usually involves calling. Special courtship flights, offers of food and mutual preening may also be involved. Like most birds and reptiles, owls lack external genitalia, with both sexes instead possessing a cloaca. This is a urogenital opening - an orifice which serves as an opening for the excretion of both urine and faeces, as well as serving reproductive purposes. During copulation, sperm are transferred via a ‘cloacal kiss’ – a brief contact of the cloacae, during which the sperm is transferred very quickly into the female reproductive tract. In some birds this can occur in less than half a second. Although two ovaries are present during the embryonic development of the female bird, the adult usually only possesses a single functional ovary. This is connected to an oviduct, through which the eggs travel, slowly becoming calcified during their journey. Sperm cannot penetrate the calcified eggshell, so must travel up the oviduct to reach the eggs before this happens. Fertilisation generally occurs in the infundibulum, a funnel-like cavity where the oviduct opens to the ovary, and each egg is fertilised individually as it is released. The number of eggs produced varies at both species and individual levels, ranging from single eggs to a dozen, although most species produce three or four.