![]() ![]() ![]() Wings, which evolved from forelimbs, gave birds the ability to fly, although further evolution has led to the loss of flight in some birds, including ratites, penguins, and diverse endemic island species. Birds have wings whose development varies according to species the only known groups without wings are the extinct moa and elephant birds. There are about ten thousand living species, more than half of which are passerine, or "perching" birds. Birds live worldwide and range in size from the 5.5 cm (2.2 in) bee hummingbird to the 2.8 m (9 ft 2 in) ostrich. In this gallery "Birds" we have 115 free PNG images with transparent background.Birds are a group of warm-blooded vertebrates constituting the class Aves /ˈeɪviːz/, characterised by feathers, toothless beaked jaws, the laying of hard-shelled eggs, a high metabolic rate, a four-chambered heart, and a strong yet lightweight skeleton. In this page you can download free PNG images: Birds PNG images free download, birds PNG Some bird species of aquatic environments, particularly seabirds and some waterbirds, have further evolved for swimming. The digestive and respiratory systems of birds are also uniquely adapted for flight. Wings, which evolved from forelimbs, gave birds the ability to fly, although further evolution has led to the loss of flight in flightless birds, including ratites, penguins, and diverse endemic island species of birds. Many of these early "stem-birds", such as Archaeopteryx, were not yet capable of fully powered flight, and many retained primitive characteristics like toothy jaws in place of beaks, and long bony tails.īirds have wings which are more or less developed depending on the species the only known groups without wings are the extinct moa and elephant birds. Primitive bird-like dinosaurs that lie outside class Aves proper, in the broader group Avialae, have been found dating back to the mid-Jurassic period, around 170 million years ago. Birds, especially those in the southern continents, survived this event and then migrated to other parts of the world while diversifying during periods of global cooling. DNA-based evidence finds that birds diversified dramatically around the time of the Cretaceous–Palaeogene extinction event 66 million years ago, which reduced the Pterosaurs, and killed off all the non-avian dinosaur lineages. True birds first appeared during the Cretaceous period, around 120 million years ago. The fossil record indicates that birds evolved from feathered ancestors within the theropod group of saurischian dinosaurs. Birds are the closest living relatives of crocodilians. ![]() They rank as the class of tetrapods with the most living species, at approximately ten thousand, with more than half of these being passerines, sometimes known as perching birds. Birds live worldwide and range in size from the 5 cm (2 in) bee hummingbird to the 2.75 m (9 ft) ostrich. They are a group of endothermic vertebrates, characterised by feathers, toothless beaked jaws, the laying of hard-shelled eggs, a high metabolic rate, a four-chambered heart, and a strong yet lightweight skeleton. Birds (Aves), a subgroup of reptiles, are the last living dinosaurs. ![]()
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