<Li> Wing - assisted incline running (WAIR), a version of "from the ground up" 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 . </Li> <Li> Pouncing proavis, which posits that flight evolved by modification from arboreal ambush tactics . </Li> <P> There has also been debate about whether the earliest known bird, Archaeopteryx, could fly . It appears that Archaeopteryx had the brain structures and inner - ear balance sensors that birds use to control their flight . Archaeopteryx also had a wing feather arrangement like that of modern birds and similarly asymmetrical flight feathers on its wings and tail . But Archaeopteryx lacked the shoulder mechanism by which modern birds' wings produce swift, powerful upstrokes; this may mean that it and other early birds were incapable of flapping flight and could only glide . The presence of most fossils in marine sediments in habitats devoid of vegetation has led to the hypothesis that they may have used their wings as aids to run across the water surface in the manner of the basilisk lizards . </P> <P> This was the earliest hypothesis, encouraged by the examples of gliding vertebrates such as flying squirrels . It suggests that proto - birds like Archaeopteryx used their claws to clamber up trees and glided off from the tops . </P>

Which of the following is true of a bird's wing during flight