Birds

The most diverse group of flyers ever to evolve are the birds (the clade Aves). Birds show a marvelous diversity not only of species but of flight adaptations - compare the hummingbird with the albatross, and you'll get a good picture of how differently animals can fly.

Flight adaptations

Birds have flight adaptations similar to those of pterosaurs:

1. hollow but strong bones, making them light-weight
2. strong, keeled sterna for flight muscle attachment making it easier to fly,
3. short and stout humeri,
4. and feathers (analogous to pterosaur wing fibers).

However, unlike the pterosaur wing, the bird wing is primarily supported by an elongated radius, ulna, and modified wrist bones (the carpometacarpus). Among other features, birds have a structure that they share with dromaeosaurs: a fused clavicle (collarbone) called the furcula (wishbone), which serves as a brace during the flight stroke. This feature was probably co- opted in function from the dromaeosaurian function of providing a brace for the shoulder girdle while holding prey.

The phalanges of the bird wing follow a pattern of reduction and fusion to form the distal part of the wing : dromaeosaurs had large clawed hands, Archaeopteryx had smaller clawed fingers and a birdlike wing, and modern birds have mostly only the second digit (finger) of the hand present (at the wing ends).

The same group also shows an evolutionary pattern of the reduction of flexion in the wrist, thus the flight stroke has become more rigid and pronounced. Later birds did not have the stiff tail of Archaeopteryx; tails seem to be structures reserved for more primitive flyers.

Birds showed a gradual increase in flying ability during their evolution --

Archaeopteryx was supposedly not a powerful flyer, but it seems it was never a glider either. Later birds such as Ichthyornis improved on the basic flight capabilities of their ancestors, becoming better flyers.

Some birds found niches that were more suited for flightless birds: Hesperornis was a flightless diving bird in the late Cretaceous period, and in the Eocene epoch period ( the time that was just after the demise of the dinosaurs), there were large flightless birds such as Diatryma that may have been the main predators on the early mammals in some areas.

From where did they evolve and when?

As is discussed in our bird origins exhibit, current theory holds that birds had a common ancestor with dromaeosaurian dinosaurs during the late Jurassic period (about 150 million years ago). Birds remained of relatively low diversity until late in the Cretaceous period (about 70 million years after their origin), when the pterosaurs were already declining in diversity, along with the dinosaurs.

Dromaeosaurs and Archaeopteryx are the two animals that are to be consider to understand the origin of flight in birds. Dromaeosaurs were all bipedal, fairly cursorial, and terrestrial. There is no truely convincing evidence indicating arboreality in dromaeosaurs. Many modern birds are arboreal, but modern birds are the the result of many years of evolution and hence are poor specimens of study for the origin of flight in birds.

Since Archaeopteryx shows basically the same features as dromaeosaurs except for the wings and feathers, the simplest explanation is that the origin of flight in birds was 'ground up' . For more information see pages regarding Archaeopteryx. But the fact that in the Solnhofen area where the original fossil bird was found does not show any evidence of trees being present. This alone does not disprove arboreality, but is one more problem with the "trees-down" model