Origin of avian flight


 * "Evolution of flight" redirects here.

Around 350 BCE, Aristotle and other philosophers of the time were attempting to explain the aerodynamics of avian flight. Even after the discovery of the ancestral bird Archaeopteryx, over 150 years ago, debates still persist regarding the evolution of flight. Currently there are three leading hypotheses pertaining to avian flight: Pouncing Proavis model, Cursorial model, and Arboreal model. Due to Archaeopteryx being the oldest known specimen of modern birds it could be the answer to the origin of avian flight.

Flight Characteristics
For flight to occur in Aves four physical forces, (thrust and drag, lift and weight) must all work together to produce flight. In order for birds to balance these forces certain physical characteristics are required. Asymmetrical wings, found on all flight birds with the exception of hummingbirds, help in the production of thrust and lift. Anything that moves produces drag due to friction forces. The aerodynamic body of a bird can reduce drag, but when stopping or slowing down a bird will use its tail and feet to increase drag. Weight is the largest obstacle birds must overcome in order to fly. Flight birds have dropped weight through evolutionary weight reducing characteristics. Pneumatic bone is bone which is hollow or composed of air sacs. These pneumatic bones are one way in which weight is reduced. The loss of teeth, gonadal hypertrophy, and fusion of bones are all ways to reduce weight. The loss of teeth have been replaced by a light weight bill made of keratin, and the chewing mechanism occurs in the birds gizzard. Other physical characteristics required for flight are a keel for the attachment of flight muscles, an enlarged cerebellum for fine motor coordination, and the presence of a furcula which enhances skeletal bracing for the stresses of flight.

Theories
The CGI television series Prehistoric Park described a theory that scales turned into contour feathers for heat insulation, and that later the feathers along the back edges of the arms and legs became bigger and longer for displaying (e.g. Incisivosaurus), until they were long enough to be used for gliding (e.g. Microraptor).

Pouncing Proavis model
This theory was first proposed by Garner, Taylor, and Thomas in 1999:

"We propose that birds evolved from predators that specialized in ambush from elevated sites, using their raptorial hindlimbs in a leaping attack. 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. Selection for enhanced lift-based control led to improved lift coefficients, incidentally turning a pounce into a swoop as lift production increased. Selection for greater swooping range would finally lead to the origin of true flight."

The authors believed that this theory had four main virtues:


 * It predicts the observed sequence of character acquisition in avian evolution.
 * It predicts an Archaeopteryx-like animal, with a skeleton more or less identical to terrestrial theropods, with few adaptations to flapping, but very advanced aerodynamic asymmetrical feathers.
 * It explains that primitive pouncers (perhaps like Microraptor) could coexist with more advanced fliers (like Confuciusornis or Sapeornis) since they did not compete for flying niches.
 * It explains that the evolution of elongated rachis-bearing feathers began with simple forms that produced a benefit by increasing drag. Later, more refined feather shapes could begin to also provide lift.

Cursorial model
This model was originally proposed by Samuel Wendell Williston in 1879. This theory states that "flight evolved in running bipeds through a series of short jumps". As the length of the jumps extended the wings were not only used for thrust but also for stability, and eventually eliminated the gliding intermediate. However, this theory was modified in the 1970s by John Ostrom to describe the use of wings as an insect foraging mechanism which then evolved into a wing stroke. Research was conducted by comparing the amount of energy expended by each hunting method with the amount of food gathered. The potential hunting volume doubles by running and jumping. To gather the same volume of food, Archaeoptryx would expend less energy by running and jumping than by running alone. Therefore, the cost/benefit ratio would be more favorable for this model. Due to Archaeopteryx long and erect leg supporters of this model say the species was a terrestrial bird. This characteristic allows for more strength and stability in the hindlimbs. Thrust produced by the wings coupled with propulsion in the legs generates the minimum velocity required to achieve flight. Thus, through these mechanisms, Archaeopteryx was able to achieve flight from the ground up.

Although the evidence in favor of this model is scientifically plausible, the evidence against it is substantial. For instance, a cursorial flight model would be less energetically favorable when compared to the alternative hypotheses. In order to achieve liftoff, Archaeopteryx would have to run faster than modern birds by a factor of three due to its weight. Furthermore, as the mass of Archaeopteryx versus the distance needed for minimum velocity to obtain liftoff speed is proportional. Therefore, as mass increases, the energy required for takeoff increases exponentially. Other research has shown that the physics involved in cursorial flight would not make this a likely answer to the origin of avian flight. Once flight speed is obtained and Archaeopteryx is in the air, drag would cause the velocity to instantaneously decrease. In addition, balance could not be maintained due to this immediate reduction in velocity. Hence, Archaeopteryx would have a very short and ineffective flight. In contrast to Ostrom’s theory regarding flight as a hunting mechanism, physics again disproves the credibility of this model. In order to effectively trap insects with the wings, Archaeopteryx would require a mechanism such as holes in the wings to reduce air resistance. Without this mechanism, the cost/benefit ratio would not be feasible.

Arboreal model
This model was originally proposed in 1880 by Othniel C. Marsh. The theory states Archaeopteryx was a reptilian bird that soared from tree to tree. After the leap, Archaeopteryx would then use its wings as a balancing mechanism. According to this model, Archaeopteryx developed a gliding method to conserve energy. Even though an arboreal Archaeopteryx exerts energy climbing the tree, an arboreal Archaeopteryx is able to achieve higher velocities and cover greater distances during the gliding phase which conserves more energy in the long run than a cursorial bipedal runner. Due to Archaeopteryx conserving energy during the gliding phase, this makes it a more energy efficient model. Therefore, the benefits gained by gliding outweigh the energy used in climbing the tree. Researchers in support of this model suggest that Archaeopteryx possessed similar skeletal features to that of modern birds. The first of these features is the similarity that exists between the foot of Archaeopteryx compared to modern birds. The hallux is a modification of the first digit, which is posterior to the remaining digits. This modification is similar to modern perching birds. Therefore, researchers have concluded that Archaeopteryx used the hallux as a balancing mechanism on tree limbs. Another skeletal feature that is similar to both Archaeopteryx and modern birds is the curvature of the claws. Archaeopteryx possessed the same claw curvature of the foot to that of perching birds. However, the claw curvature of the hand in Archaeopteryx was similar to basal birds. Based upon the comparisons of modern birds to Archaeopteryx, perching characteristics were present signifying an arboreal habitat. The ability for takeoff and flight was originally thought to require a supracoracoideus pulley system (SC). This system consists of a tendon joining the humerus and coracoid process of the scapula allowing rotation of the humerus during the upstroke. However, this system is lacking in Archaeopteryx. Based on experiments performed by Sy, it was proven that the SC pulley system was not required for flight from an elevated position but necessary for cursorial takeoff.