Origin of avian flight
Even after the discovery of the ancestral bird Archaeopteryx which lived over 150 million years ago, debates still persist regarding the evolution of flight.
Asymmetrical wing feathers, found on all flying birds with the exception of hummingbirds, help in the production of thrust and lift.
Birds evolved from other theropod dinosaurs that had already gone through a phase of size reduction during the Middle Jurassic, combined with rapid evolutionary changes.
[3] Flying birds during their evolution further reduced relative weight through several characteristics such as the loss of teeth, shrinkage of the gonads out of mating season, and fusion of bones.
These were gradual changes, though, and not strict conditions for flight: the first birds had teeth, at best a small keel and relatively unfused bones.
Pneumatic bone, that is hollow or filled with air sacs, has often been seen as an adaptation reducing weight, but it was already present in non-flying dinosaurs, and birds on average do not have a lighter skeleton than mammals of the same size.
These functions depend on a precise balance of forces from the muscles, ligaments, and articular cartilages as well as inertial, gravitational, and aerodynamic loads on the wing.
Drag-based, and later lift-based, mechanisms evolved under selection for improved control of body position and locomotion during the aerial part of the attack.
The authors believed that this theory had four main virtues: A cursorial, or "running" model was originally proposed by Samuel Wendell Williston in 1879.
Thrust produced by the wings coupled with propulsion in the legs generates the minimum velocity required to achieve flight.
[12] The WAIR hypothesis, a version of the "cursorial model" of the evolution of avian flight, in which birds' wings originated from forelimb modifications that provided downforce, enabling the proto-birds to run up extremely steep slopes such as the trunks of trees, was prompted by observation of young chukar chicks, and proposes that wings developed their aerodynamic functions as a result of the need to run quickly up very steep slopes such as tree trunks, for example to escape from predators.
They begin with wing-assisted incline running and slowly alter their wing strokes for flight as they grow and are able to make enough force.
[19] Wing-assisted incline running can also produce a useful lift in babies but is very small compared to that of juveniles and adult birds.
Dudley and Yanoviak proposed that animals that live in trees generally end up high enough that a fall, purposeful or otherwise, would generate enough speed for aerodynamic forces to have an effect on the body.
Many animals, even those which do not fly, demonstrate the ability to right themselves and face the ground ventrally, then exhibiting behaviors that act against aerodynamic forces to slow their rate of descent in a process known as parachuting.
[22] Arboreal animals that were forced by predators or simply fell from trees that exhibited these kinds of behaviors would have been in a better position to eventually evolve capabilities that were more akin to flight as we know them today.
However, study of the Thermopolis specimen of Archaeopteryx, which has the most complete foot of any known, showed that the hallux was not in fact reversed, limiting the creature's ability to perch on branches and implying a terrestrial or trunk-climbing lifestyle.
Based on experiments performed by M. Sy in 1936,[24] it was proven that the SC pulley system was not required for flight from an elevated position but was necessary for cursorial takeoff.
Some researchers have suggested that treating arboreal and cursorial hypotheses as mutually exclusive explanations of the origin of bird flight is incorrect.
[25] Researchers in support of synthesizing cite studies that show incipient wings have adaptive advantages for a variety of functions, including arboreal parachuting, WAIR, and horizontal flap-leaping.
[26] Other research also shows that ancestral avialans were not necessarily exclusively arboreal or cursorial, but rather lived on a spectrum of habitats.
The capability for powered flight evolved due to a multitude of selective advantages of incipient wings in navigating a more complex environment than previously thought.
