Final answer:
Birds are part of the Archosaur clade, evolved from maniraptoran theropod dinosaurs, and constitute a unique group of endothermic vertebrates with more than 10,000 species.
Step-by-step explanation:
From an evolutionary perspective, birds are part of the Archosaur clade. As descendants of theropod dinosaurs, birds are located within the broader grouping of diapsids. Specifically, they evolved from the maniraptoran theropods about 150 million years ago. This evolution is supported by numerous anatomical similarities found in fossil records, such as the structure of hip and wrist bones, the presence of a wishbone formed by the fusion of the clavicles, and features such as feathers which originated from these dinosaur ancestors. Birds exhibit many unique adaptations including endothermy, feathers, and flight capabilities that distinguish them from their reptile relatives.
Birds comprise over 10,000 species, making them highly successful and conspicuous vertebrates that occupy a variety of ecological niches. Their evolution is underpinned by remarkable fossil discoveries, particularly from areas like China, which have provided significant insights into their ancestry. The classification of modern birds includes groups such as Paleognathae, Galloanserae, and Neoaves. Contrary to the historic classification of animals into Mammalia, Reptilia, and Aves, recent research identifies birds as a thriving clade within the Reptilia, more specifically within the archosaurs.