Birds are a group of warm-blooded vertebrates of the class Aves , defined by feathers, toothless beaked jaws, the laying of hard-shelled eggs, a rapid metabolic rate, a four-chambered heart, and a robust yet lightweight skeleton. Birds are found all throughout the world and come in a variety of sizes, from the 2.2 in (5.5 cm) bee hummingbird to the 9 ft 2 in (2.8 m) common ostrich. More than half of the approximately 10,400 species that are now extant are passerine, or “perching” birds. The evolution of a bird’s wings differs from species to species; the only known bird species without wings are the extinct moa and elephant birds.
The capacity to fly was bestowed to birds via their wings, which are modified forelimbs. However, subsequent evolution has caused certain birds—such as ratites, penguins, and a variety of unique island species—to lose their ability to fly. Birds’ respiratory and digestive systems are also specially designed for flying. Certain aquatic bird species, including seabirds and certain waterbirds, have undergone further evolution to enhance their swimming abilities. We refer to the study of birds as ornithology.
The only known extant dinosaurs are birds, which are feathered theropod dinosaurs. In the same way, birds are categorized as reptiles in the contemporary cladistic sense, with crocodilians being their closest living cousins. The first avialans, which included Archaeopteryx, originally emerged in the Late Jurassic and are the ancestors of birds. Based on DNA evidence, modern birds (Neornithes) diverged drastically at the time of the Cretaceous–Paleogene extinction event (66 mya), which wiped out all non-avian dinosaurs and pterosaurs. Modern birds emerged in the Early to Late Cretaceous.