The digestive and respiratory systems of birds are also adapted for flight. Later, many groups evolved with reduced wings, such as ratites, penguins and many island species of birds. Wings, which evolved from forelimbs, gave birds the ability to fly. The only known groups without wings are the extinct moa and elephant birds. īirds have wings which are more or less developed depending on the species. Diversification occurred around the Cretaceous–Palaeogene extinction event. Birds, especially those in the southern continents, survived this event and then migrated to other parts of the world. The Cretaceous–Palaeogene extinction event 66 million years ago killed off all the non-avian dinosaur lines. Many had primitive characteristics like teeth in their jaws and long bony tails. Many of these early "stem-birds", such as Anchiornis, were not yet capable of fully powered flight. They have been found back to the mid-Jurassic period, around 170 million years ago. Primitive bird-like dinosaurs are in the broader group Avialae. More recent estimates showed that modern birds originated early in the Upper Cretaceous. According to DNA evidence, modern birds (Neornithes) evolved in the long Upper Cretaceous period. Modern birds are not descended from Archaeopteryx. This is because they are the two main survivors of a once huge group called the Archosaurs. More than half of these are passerines, sometimes known as perching birds.īirds are the closest living relatives of the Crocodilia. They are the tetrapods with the most living species: about ten thousand. They range in size from the 5 cm (2 in) bee hummingbird to the 2.70 m (9 ft) ostrich. They have a high metabolic rate, a four-chambered heart and a strong yet lightweight skeleton.īirds live all over the world. Modern birds are toothless: they have beaked jaws. The heat loss from their bodies is slowed down by their feathers.
Technically speaking, they are dinosaurs.
Birds ( Aves) are a group of animals with backbones which evolved from dinosaurs.
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