Complex sentence: Birds (class Aves and clade Avialae) are highly advanced theropod dinosaurs, characterised by feathers, a beak with no teeth, the laying of hard-shelled eggs, a high metabolic rate, a four-chambered heart, and a lightweight but strong skeleton.
Split into simpler sentences: Birds (class Aves and clade Avialae) are highly advanced theropod dinosaurs. Birds are characterised by feathers, a beak with no teeth, the laying of hard-shelled eggs, a high metabolic rate, a four-chambered heart, and a lightweight but strong skeleton.