Most birds do not have the same reproductive body parts as mammals. Instead, both male and female birds have a cloaca – one opening (also called the vent) that serves as the bodily exit for their digestive, urinary and reproductive systems. This means that the same opening that excretes feces and urine is where eggs are laid. During the breeding season, the cloaca swells and protrudes slightly outside the body, while during the rest of the year it is much less prominent.

When birds are ready to breed, their reproductive organs – the testes and ovaries – swell and produce the sperm and ova. Male birds store sperm in their cloaca until an opportunity to mate arises, and females will receive that sperm into their cloaca before it travels to fertilize their ova.