Boundaries among birds, reptiles, and mammals seem clear-cut when only living species are considered. All living members of Reptilia, but no mammals, are capable of producing uric acid as a means of expelling nitrogenous waste. This provides two major benefits for terrestrial amniotes. Uric acid is insoluble and can safely build up in the allantois. Almost all living mammals are descendants of a lineage that developed live birth.
Monotremes are the exceptions.
Because they are not as well adapted to life inside the amniotic egg, most surviving synapsids are those that give birth to live young. Most reptiles, including birds, continue to lay eggs. Additionally, no close relatives of synapsids have been found which lack lower temporal fenestrae.