Which living reptiles are the closest living relatives of birds?

Crocodilians are the closest living relatives of birds among living reptiles.

Crocodilians include crocodiles, alligators, caimans, and gharials. Birds and crocodilians are the only surviving members of Archosauria, the ancient reptile group that also included dinosaurs and pterosaurs. Their shared ancestry is visible in features such as parts of the skull, heart and circulatory anatomy, eggshell structure, and aspects of breathing and reproduction.

This relationship means that a crocodile is more closely related to a bird than it is to a lizard, snake, or turtle. Birds themselves are living dinosaurs, specifically surviving theropod dinosaurs, while crocodilians belong to the archosaur branch that includes crocodile-line reptiles.

A common mistake is to use “crocodiles” for every member of the group. Crocodiles are one family, Crocodylidae; the broader term “crocodilians” also includes alligators, caimans, and gharials. Turtles are also ancient reptiles and may be relatively close to archosaurs in some evolutionary analyses, but crocodilians are the accepted closest living relatives of birds because they share a direct sister-group relationship with them.