Roosters mate with hens by a process called cloacal kissing. This is when the male presses his vent, which contains his reproductive organ, against the hen’s vent. The vents of both animals touch and the male pass sperm into the female reproductive system. The cloaca is the common opening for the digestive, urinary, and reproductive tracts in birds. Chickens do not have external genitalia like mammals, so cloacal kissing is the only way for roosters to mate with hens. This usually happens while the hen is perched on a perch or on the ground. The rooster will mount her from behind and align his cloaca with hers. Once they are touching, they will begin to rub their vents together and exchange sperm. Cloacal kissing can be a violent process, and sometimes hens can be injured or killed by aggressive roosters. For this reason, many farms will only allow one rooster for every ten hens