Flightless birds are birds that, through evolution, lost the ability to fly. There are over 60 extant species, including the well-known ratites and penguins. The smallest flightless bird is the Inaccessible Island rail. The largest flightless bird, which is also the largest living bird in general, is the common ostrich.
King penguins (Aptenodytes patagonicus). Penguins are a well-known example of flightless birds.
An Okarito kiwi (Apteryx rowi), also known as the rowi
Common ostrich (Struthio camelus). Ostriches are the largest extant flightless birds as well as the largest extant birds in general.
An extinct moa. Until the arrival of humans, New Zealand's only mammals were bats and seals, resulting in many bird species evolving to fill the open niches. While many of New Zealand's flightless birds are now extinct, some, such as the kiwi, kākāpō, weka, and takahē have survived to the present day.
Birds are a group of warm-blooded vertebrates constituting the class Aves, 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 live worldwide and range in size from the 5.5 cm (2.2 in) bee hummingbird to the 2.8 m common ostrich. There are over 11,000 living species, more than half of which are passerine, or "perching" birds. Birds have wings whose development varies according to species; the only known groups without wings are the extinct moa and elephant birds. Wings, which are modified 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. The digestive and respiratory systems of birds are also uniquely adapted for flight. Some bird species of aquatic environments, particularly seabirds and some waterbirds, have further evolved for swimming. The study of birds is called ornithology.
Archaeopteryx is often considered the oldest known true bird.
Anchiornis huxleyi is an important source of information on the early evolution of birds in the Late Jurassic period.
Confuciusornis sanctus, a Cretaceous bird from China that lived 125 million years ago, is the oldest known bird to have a beak.
The range of the house sparrow has expanded dramatically due to human activities.