A Bird is a warm-blooded egg-laying vertebrate with wings and feathers. For the most part, they fly. Ostriches, penguins, and emus are birds that don't fly, although penguins have sometimes been said to "fly" underwater. Robins, sparrows, cardinals, and eagles are more typical flying birds.
All birds have two legs and two wings (although in some flightless species such as the kiwi, the wings are very small and serve little purpose). Birds also have a beak, although its shape, size and use varies enormously between species. Uniquely among vertebrates, birds have hollow bones; this makes them very light for their size, facilitating flight. Living birds do not have teeth, although some extinct species had teeth. Birds' feathers can be any color, and the colors and pattern often vary between males and females of the same species, with males typically having the more garish and elaborate plumage; in some species such as peacocks and birds of paradise this is taken to striking extremes.
Birds as dinosaurs
Almost all evolutionary biologists believe that birds are the only extant group of dinosaurs, a branch of the therapods, which included Velociraptor and Deinonychus. This view is disputed by creationists, and also by a few prominent evolutionary biologists. Some have even gone so far as to suggest that dinosaur species such as the Velociraptor and others possessed primitive feathers.
They also point out that God could have created some dinosaurs with feathers, and therefore that finding feathered dinosaurs does not prove that dinosaurs evolved into birds.
Creationary scientist Dr. Jonathan Sarfati wrote regarding birds being descendants of dinosaurs:
The same logic applies to the dinosaur-bird debate. It is perfectly in order for creationists to cite Feduccia’s devastating criticism against the idea that birds evolved ‘ground up’ from running dinosaurs (the cursorial theory). But the dino-to-bird advocates counter with equally powerful arguments against Feduccia’s ‘trees-down’ (arboreal) theory. The evidence indicates that the critics are both right — birds did not evolve either from running dinos or from tree-living mini-crocodiles. In fact, birds did not evolve from non-birds at all!
Evolutionary biologist Ernst Mayr in 1942 wrote:
It must be admitted, however, that it is a considerable strain on one’s credulity to assume that finely balanced systems such as certain sense organs (the eye of vertebrates, or the bird’s feather) could be improved by random mutations.
Scientific American stated:
Of all the body coverings nature has designed, feathers are the most various and the most mysterious...The origin of feathers is a specific instance of the much more general question of the origin of evolutionary novelties--structures that have no clear antecedents in ancestral animals and no clear related structures (homologues) in contemporary relatives. Although evolutionary theory provides a robust explanation for the appearance of minor variations in the size and shape of creatures and their component parts, it does not yet give as much guidance for understanding the emergence of entirely new structures, including digits, limbs, eyes and feathers...Another evolutionist critical of the dinosaur/bird connection is Storrs L. Olson, Curator of Birds at the National Museum of Natural History at the Smithsonian Institution, who said:
The idea of feathered dinosaurs and the theropod origin of birds is being actively promulgated by a cadre of zealous scientists acting in concert with certain editors at Nature and National Geographic who themselves have become outspoken and highly biased proselytizers of the faith. Truth and careful scientific weighing of evidence have been among the first casualties in their program, which is now fast becoming one of the grander scientific hoaxes of our age—the paleontological equivalent of cold fusion.
Since Olsen's open letter was published in 1999, however, numerous theropod fossils with clear evidence of feathers have been discovered.
Creationist also assert that the comparative anatomy analysis done by evolutionists comparing bird bones and dinosaur bones is flawed.
A widespread evolutionist claim is that birds evolved from earlier theropod dinosaurs over 150 million years ago. The most-frequently claimed candidate for a transitional form is Archaeopteryx lithographica which is from the late Jurassic (about 150 million years ago) of Germany. This is a flawed example, as creationists often point out that God could have just created a feathered dinosaur. There are other claimed transitional fossils  also.
There are many papers in the peer-reviewed science literature, including leading journals such as Nature and Science, that support the idea that birds are a sub-group of dinosaurs. For example:
Several features, including a carinate sternum and reduced fibula, suggest that Mononychus olecranus is more closely related to modern birds than is Archaeopteryx lithographica. The two skeletons are among the best preserved fossils known of a primitive bird, and emphasize the complexity of the morphological transformation from nonavialian theropods to modern birds. The occurrence of such a primitive bird in the Late Cretaceous reflects the paucity of Mesozoic bird fossils and suggests that the early radiation of avialians is only beginning to be sampled 
Since those words were written in 1993, however, many fossils of primitive birds and other theropods have been discovered, so it is no longer correct to consider the Late Mesozoic fossil evidence of this group as depauperate.
The conventional Linnaean taxonomy places birds in the Class Aves within the Subphylum Vertebrata. Reptilia, the conventional Class of reptiles, includes crocodiles and relatives, lizards and snakes, turtles, and the tuatara. However, birds and other, extinct dinosaurs should be grouped as archosaurs, along with the extant crocodiles and the extinct pterosaurs. Hence the conventional classification does not reflect the current state of evolutionary biology.
Zoologists continue to use Class Aves as the taxonomic group of birds but recognise that it is a sub-group of the Superorder Dinosauria, which in turn is a member of the group Archosauria, including crocodiles and pterosaurs. The higher classification of reptiles (including birds) is not fully resolved but a common view is that current evidence indicates that the archosaurs (including birds) are a sister group of the lepidosaurs, which include lizards, snakes, the tuatara, and the extinct mosasaurs and plesiosaurs.
Evolution of flight
Proposed in 1879 by fossil expert Samuel Williston, the Cursorial Theory suggests that the ancestors of birds were ground-dwelling theropods that developed flight through leaping and jumping. Cursorial Theory is no longer popular among scientists, who generally favor the Arboreal "trees down" theory of flight, since taking off from the ground is energetically expensive, thus it is therefore considered unlikely that flapping flight evolved from the ground up.
However, proponents of the Cursorial theory point to the basilisk lizard, which possesses the ability to run on water for a short time to escape predators, yet also has the ability to climb with great agility. The ability to run bipedally is not evident from the basilisk anatomy alone, and this illustrates the plasticity of animal behavior. It is nearly impossible to determine an animal's behavior simply by looking at its anatomy, as animals are capable of behavior beyond what their anatomy would suggest.
- Feather Quill Knobs in the Dinosaur Velociraptor Science 21 September 2007: Vol. 317. no. 5845, p. 1721 http://www.sciencemag.org/cgi/content/full/317/5845/1721
- Sarfati, Jonathan, Bird Evolution, chapter 4 of Refuting Evolution.
- McIntosh, Andy, 100 years of airplanes—but these weren’t the first flying machines!, Creation 26(1):44–48, December 2003.
- Matthews, Michael, Scientific American admits creationists hit a sore spot 13th March, 2003 (Creation Ministries International)
- Sarfati, Jonathan, 15 ways to refute materialistic bigotry: A point by point response to Scientific American, Creation Ministries International
- Brown, Walt, In the Beginning: Compelling Evidence for Creation and the Flood
- Matthews, Michael, Scientific American admits creationists hit a sore spot, Answers in Genesis.
- Prum, Richard O., and Brush, Alan H., Which Came First, the Feather or the Bird?, Scientific American, March 2003.
- Olson, Storrs L., Open letter, 1st November, 1999.
- Göhlich, U.B., and Chiappe, L.M. (2006). "A new carnivorous dinosaur from the Late Jurassic Solnhofen archipelago." Nature, 440: 329-332
- Menton, David N., "Ostrich-osaurus" discovery?, Answers in Genesis.
- Nature 362, 623-626 (15 April 1993)
- Benton, 2004. Vertebrate Paleontology