Birds have developed an original reproductive process that allows an efficient transmission of the genome of these flying vertebrates in highly changing environments. This original process includes the production by the heterogametic female (ZW) of telolecithal hard-shelled eggs by a most often unique female tract with an ovary containing a strict follicular hierarchy. The presence of sperm storage tubules in the oviduct allows quite long conservation of the sperm produced in high number by the two internal testis of the homogametic male (ZZ). These adaptations have been important for the development of poultry productions.