I'm Arthur H. Tarver, [email protected]
Some years ago, (2003), I concluded that the ground-up and the tree-down theories for the origin of bird flight were both wrong. The ground-up senarios are all physicaly impossible because any deployment of feathered forelimbs while running will result in more drag than lift or thrust. Tree-down won't work because bipedal theropods didn't climb trees and aboreal vertebrates need four climbing limbs to live there.
So, I wrote an essay to explain how it really happend. You can read it at protobird.blogspot.com.
This is a condensed version how and why bird flight evolved.
My theory proposes that small bipedal dinosaurs (theropods) were living a semi-aquatic life along rivers, lakes and swamps. They were warm-blooded and had feathers that were used for warmth, sunshade, brooding eggs and for buoyancy to aid in swimming. They waded, foraged or stalked prey in shallow water. They would run and jump feet first to take fish, amphibians, reptiles, etc. Their warm bloodedness would allow them to attack at dawn. They would use their hind leg claws to seize and immobilize prey and their teeth to tear them apart to feed to their young. Later, they jumped from rocks, banks and cliffs head first to capture small fish with their mouths directly. They would use their forelimbs to swim back to shore. Most could succeed from the low rocks. Less would succeed from the high overhangs. Those that succeeded had feathered tufted tails that would be used to stay straight, or within a cone of movement, produce some yaw or pitch to better target moving fish. A favored race would separate and begin to utilize their feathered forelimbs for attitude control. Their forelimbs and tails would change to aerodynamic fins with the emergence of flight feathers. Quicker pitch and roll and eventually lift will allow them to extend their range. Flapping their forelimbs fins would extend their glide slope. They could dive in head first, belly in or bring their feet forward to seize prey. As they came close to flat water they could utilize the air-cushion affect to glide level. Eventually a fully articulating tail and asymmetrical feathers would allow them to glide up, down or over the river. Integrated anatomical changes will allow them to flap and lift upward to exploit the terrain and greatly extend their hunting range. They would glide up and down river valleys using ridge lift to explore new areas. This mobility would greatly enhance their survival. They could nest far from where they hunted, and they could move seasonally. This theory explains the first exaptation, the tail, from balance while standing, running and turning, to an aerodynamic steering device. This theory has no risky trial and error scenarios where failure is dangerous, and natural selection would reject. This theory doesn’t need to explain how bipedal animals climbed trees. This theory does not depend on incipient flight feathers without explaining how they came about. This theory doesn't need to ignore that four legged running would serve attack or defense much more effectively than feeble forelimb flapping assist. This theory's initial aerodynamic act, diving for fish, is still performed by some modern birds today.