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Like the males of 97 percent of all bird species, a rooster does not have a penis. An incubated egg that will become a rooster starts to develop a penis, but early in the second week of embryonic development, a cell death protein called Bmp4 cloaks the incipient penis, causing it to stop developing and instead remain as a rudimentary nub.
So how do chicken eggs get fertilized?
The rooster and hen kiss!
But not with their beaks.
The rooster injects sperm into a hen by pressing his vent against hers in a process known as the cloacal kiss. The cloaca is the chamber just inside the vent where the digestive, urinary, and reproductive tracts end.
The rooster’s cloaca contains two tiny nipples (papillae) at the ends of the two ducts that transport semen from the rooster’s testes. The testes are hidden inside the rooster, near his backbone, and look like a pair of large white beans. To transfer semen into a hen, the two cloacal papillae serve as the rooster’s mating organs.