Both male and female birds have what is known as cloaca. This is an internal chamber and through this opening, a bird's sex organ - testes or ovaries - discharge sperm or eggs. The same opening also is used for expulsion of urinary and digestive waste. During mating seasons, the cloacal openings swell, protruding outside the bird's body.
In this mating of Caspian Terns shown in the photos below, the male is on top of the female. The female opens her bill and snaps it shut. The female then lifts her tail high to expose her rear and her cloacal opening The male arches back, rubs his cloaca against hers so that his sperm is deposited into her cloacal opening. The male's sperm travels up the female's chamber and fertilizes the eggs. The female grabs at her partners bill and than the male lifts out. This delicate balancing act can sometimes take several attempts before resulting in successful copulation.
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