Birds and reptiles share a number of characteristics, including scaly integuments, skeletal system details and methods of reproduction. Because birds descended from a group of theropod dinosaurs, they are in fact reptiles themselves. However, the reptile family tree is quite broad, and birds are more similar to some reptiles than they are to others.
Keratinous scales cover the bodies of most non-avian reptiles, whereas feathers cover the bodies of birds. However, keratinous scales cover the legs of birds, providing yet another clue of their reptilian ancestry. Just as reptiles shed their scales when weathered, worn or outgrown, so do birds.
All living members of the reptilian family tree – including birds – bear a number of subtle skeletal details. For example, the skulls of both birds and non-avian reptiles attach to the neck vertebrae at a single point called te occipital condyle. This single point of attachment allows some birds and reptiles to rotate their heads farther than mammals can, as mammals have two occipital condyles. Other similarities between non-avian reptiles and birds include the presence of a single middle ear bone, rather than the set of three middle ear bones common to most mammals. Additionally, the lower jawbones of both birds and non-avian reptiles are made of five fused bones rather than of one solid bone as in mammals.
While all crocodiles, tuataras and most squamates (a clade consisting of snakes, lizards and amphisbaenians) have teeth, all living birds and turtles possess toothless beaks for mouths. Technically called rhamphothecae, their beaks are built from bone and overlaid with keratin. Millions of years ago, some turtles and birds possessed teeth. In fact, Archaeopteryx – the oldest known fossil that is definitively a bird – had a mouth full of teeth.
All birds, crocodiles, turtles and tuataras reproduce by depositing eggs – a reproductive mode called oviparity. Most snakes and lizards are also oviparous, although a few produce unshelled eggs and retain them internally during the process of development (a process called ovoviviparity). Aside from ovoviviparous snakes and lizards, birds and their reptilian kin coat their eggs in calcium, which reduces the amount of water that can pass through the shell. The mineral content of the eggshell varies from one species to the next, enabling some to produce flexible, leathery eggs and others to produce hard eggs.
The development of the shelled egg enabled primitive reptiles to colonize terrestrial habitats. It also means that all reptiles – avian and non-avian alike – must practice internal fertilization, and they must do so before calcium is deposited around the eggs.
To cut their way out of the eggs, young birds and other reptiles bear small, sharp structures known as "egg teeth," which fall off shortly after they hatch.
Biologists recognize two primary reptile lineages (called clades): Archosauria, which contains the living birds and crocodiles, and Lepidosauria, which contains snakes, lizards and tuataras. Scientists have long debated the placement of turtles within the reptilian tree of life; however, a 2014 study led by Nicholas G. Crawford of the California Academy of Science in San Francisco, California, may have settled the debate. The multi-institutional team, who published their results in the journal "Molecular Phylogenetics and Evolution," determined that turtles emerged from within the Archosaurian clade. This means that turtles and crocodilians are the closest living relatives of birds.
Birds and all other living reptiles possess one interesting circulatory similarity: Their red blood cells (erythrocytes) contain nuclei. By contrast, humans and other mammals have non-nucleated red blood cells. However, few other circulatory similarities exist between birds and other reptiles.
Whereas non-avian reptiles bear three-chambered hearts that allow partial mixing of oxygenated and deoxygenated blood, birds have mammal-like circulatory systems, composed of two distinct circuits and driven by four-chambered hearts. Crocodilians also have four-chambered hearts, but they also have the small feature called the Foramen of Panizza that allows some mixing of oxygenated and deoxygenated blood.