There are about ten thousand 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 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. 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.
Best of all, birds are feathered theropod dinosaurs and constitute the only living dinosaurs. Based on fossil and biological evidence, most scientists accept that birds are a specialized subgroup of theropod dinosaurs, and more specifically, they are members of Maniraptora, a group of theropods which includes dromaeosaurs and oviraptorids, amongst others. As palaeontologists discover more theropods closely related to birds, the previously clear distinction between non-birds and birds has become a bit muddy.
Recent discoveries in the Liaoning Province of northeast China, which include many small theropod feathered dinosaurs — and some excellent fakes — contribute to this ambiguity. Still, other fossil specimens found here shed a light on the evolution of Aves. Confuciusornis sanctus, an Early Cretaceous bird from the Yixian and Jiufotang Formations of China is the oldest known bird to have a beak.
Like modern birds, Confuciusornis had a toothless beak, but close relatives of modern birds such as Hesperornis and Ichthyornis were toothed, telling us that the loss of teeth occurred convergently in Confuciusornis and living birds.
|Confuciusornis sanctus, Cretaceous Bird from China, 125 mya|
Together, these form a group called Paraves. Some basal members of this group, such as Microraptor, have features which may have enabled them to glide or fly. The most basal deinonychosaurs were wee little things. This evidence raises the possibility that the ancestor of all paravians may have been arboreal, have been able to glide, or both. Unlike Archaeopteryx and the non-avialan feathered dinosaurs, who primarily ate meat, tummy contents from recent avialan studies suggest that the first avialans were omnivores. Even more intriguing...
Avialae or "bird wings" are a clade of flying dinosaurs containing the only living dinosaurs, the birds. It is usually defined as all theropod dinosaurs more closely related to modern birds — Aves — than to deinonychosaurs, though alternative definitions are occasionally bantered back and forth.
Archaeopteryx lithographica, from the late Jurassic Period Solnhofen Formation of Germany, is the earliest known avialan which may have had the capability of powered flight. However, several older avialans are known from the Late Jurassic Tiaojishan Formation of China, dating to about 160 million years ago.
The Late Jurassic Archaeopteryx is well-known as one of the first transitional fossils to be found, and it provided support for the theory of evolution in the late 19th century. Archaeopteryx was the first fossil to clearly display both traditional reptilian characteristics — teeth, clawed fingers, and a long, lizard-like tail—as well as wings with flight feathers similar to those of modern birds. It is not considered a direct ancestor of birds, though it is possibly closely related to the true ancestor.
Unlikely yet true, the closest living relatives of birds are the crocodilians. Birds are descendants of the primitive avialans — whose members include Archaeopteryx — which first appeared about 160 million years ago in China.
In birds, the brain, especially the telencephalon, is remarkably developed, both in relative volume and complexity. Unlike most early‐branching sauropsids, the adults of birds and other archosaurs have a well‐ossified neurocranium. In contrast to most of their reptilian relatives, but similar to what we see in mammals, bird brains fit closely to the endocranial cavity so that major external features are reflected in the endocasts. What you see on the inside is what you see on the outside.
This makes birds an excellent group for palaeoneurological investigations. The first observation of the brain in a long‐extinct bird was made in the first quarter of the 19th century. However, it was not until the 2000s and the application of modern imaging technologies that avian palaeoneurology really took off.
Understanding how the mode of life is reflected in the external morphology of the brains of birds is but one of several future directions in which avian palaeoneurological research may extend.
Although the number of fossil specimens suitable for palaeoneurological explorations is considerably smaller in birds than in mammals and will very likely remain so, the coming years will certainly witness a momentous strengthening of this rapidly growing field of research at the overlap between ornithology, palaeontology, evolutionary biology and the neurosciences.
Reference: Cau, Andrea; Brougham, Tom; Naish, Darren (2015). "The phylogenetic affinities of the bizarre Late Cretaceous Romanian theropod Balaur bondoc (Dinosauria, Maniraptora): Dromaeosaurid or flightless bird?". PeerJ. 3: e1032. doi:10.7717/peerj.1032. PMC 4476167. PMID 26157616.
Reference: Ivanov, M., Hrdlickova, S. & Gregorova, R. (2001) The Complete Encyclopedia of Fossils. Rebo Publishers, Netherlands. p. 312
Photo: By Tommy from Arad - Confuciusornis; FunkMonk, CC BY 2.0, https://commons.wikimedia.org/w/index.php?curid=24115307