Around 350 BCE, Aristotle and other philosophers of the time attempted to explain the aerodynamics 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. There are three leading hypotheses pertaining to avian flight. In the Pouncing Proavis model, it is assumed to have evolved from ambush predators pouncing on prey from above. The Cursorial model assumes that flight started with running dinosaurs making short leaps and evolving proto-wings for greater control over those leaps. The third hypothesis, the Arboreal model, assumes birds evolved from tree-dwelling gliders, who gradually increased their control and flight distance. In March 2018, scientists reported that Archaeopteryx was likely capable of flight, but in a manner substantially different from that of modern birds.
Flight characteristics For flight to occur, four physical forces (thrust and drag, lift and weight) must be favorably combined. In order for birds to balance these forces, certain physical characteristics are required. Asymmetrical wing feathers, found on all flying birds with the exception of hummingbirds, help in the production of thrust and lift. Anything that moves through the air produces drag due to friction. 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. An animal can more easily attain flight by reducing its absolute weight. 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. 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. Teeth were replaced by a lightweight bill made of keratin, the food being processed by the bird's gizzard. Other advanced physical characteristics evolved for flight are a keel for the attachment of flight muscles and an enlarged cerebellum for fine motor coordination. 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, and was already present in non-flying dinosaurs, and birds on average do not have a lighter skeleton than mammals of the same size. The same is true for the furcula, a bone which enhances skeletal bracing for the stresses of flight. Birds have denser bones than both flying and non-flying mammals of a similar size, meaning birds have the strongest skeletons for their weight of the three. The mechanics of an avian's wings involve a complex interworking of forces, particularly at the shoulder where most of the wings' motions take place. 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. Birds have two main muscles in their wing that are responsible for flight: the pectoralis and the supracoracoideus. The pectoralis is the largest muscle in the wing and is the primary depressor and pronator of the wing. The supracoracoideus is the second largest and is the primary elevator and supinator. In addition, there are distal wing muscles that assist the bird in flight. Prior to their existence on birds, feathers were present on the bodies of many dinosaur species. Through natural selection, feathers became more common among the animals as their wings developed over the course of tens of millions of years. The smooth surface of feathers on a bird's body helps to reduce friction while in flight. The tail, also consisting of feathers, helps the bird to maneuver and glide.
Hypotheses
Pouncing Proavis model A theory of a pouncing proavis 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.
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