in clades that have lost some digits in the course of their evolution, and therefore bird's hands do develop from digits 1, 2 and 3. Wing-assisted incline running The wing-assisted incline running (WAIR) hypothesis 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. Note that in this scenario birds need downforce to give their feet increased grip. But early birds, including Archaeopteryx, lacked the shoulder mechanism that modern birds' wings use to produce swift, powerful upstrokes. Since the downforce that WAIR requires is generated by upstrokes, it seems that early birds were incapable of WAIR. Pouncing proavis model The proavis 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. Uses and loss of flight in modern birds Birds use flight to obtain prey on the wing, for foraging, to commute to feeding grounds, and to migrate between the seasons. It is also used by some species to display during the breeding season and to reach safe isolated places for nesting. Flight is more energetically expensive in larger birds, and many of the largest species fly by soaring and gliding (without flapping their wings) as