Reproduction in the Female

The avian lineage evolved from ancestral, oviparous amniotes that include reptiles and the archosaurs (therapod dinosaurs and crocodilians). Oviparity represents the ancestral form of reproduction in all archosaurs, and it is the only form of reproduction utilized by birds. Whereas all modern crocodilians have two functional ovaries, only the left ovary is functional in the majority of avian species. Significantly, avian fossils from two species of an extinct lineage (enantiornithine birds of the Early Cretaceous period) have revealed that these ancestors possessed a single, functional left ovary. This is consistent with the hypothesis that the loss of one ovary to reduce body weight occurred early in avian evolution, perhaps even preceding the capacity for true flight. Interestingly, viviparity has evolved nearly 100 times within the reptilian lineage that includes lizards and snakes; thus, it is unclear why there is no incidence of viviparity within the avian lineage. One line of reasoning is that because extant birds are endothermic and can precisely control the process of egg incubation and embryo development, there may be no thermoregulatory advantage to viviparity in birds.