At first blush, their reproductive organs seem all alike. Both male and female have a cloaca, an opening that in the male swells during the mating season, projecting outside their bodies. When birds mate, they briefly rub together their swollen cloacae, allowing the male’s sperm to move from his cloaca to hers and then travel up her reproductive tract to fertilize her egg. (Bird cloacae also have a decidedly less sexy role: to excrete urine and feces.)
