I believe that birds evolve from dinosaurs not through the process of adaptation, but through ritualistic selection. Just like some birds select their breeding pair through ritualistic displays, the same way some dinosaurs were able to select their mates millions of years ago.
I believe that some male dinosaurs showed and displayed their colorful scale like skin to the females for the purpose of mating, until one day through mutations some dinosaurs within a specie began showing very colorful shiny scale like skin in some parts of the body, such as near the neck, on top of their heads and other places. Their skin was shiny for it had little ripples that when the light shined on them, it looked a bit metallic just like some birds feathers reflect the light in the same manner. This attracted the female dinosaurs into mating with these individuals more than the others, therefore, improving the quality of this genetic trait, until these ripples eventually became tiny slender furry like scales within their scale like skin.
Time went by, and these scales became more and more furry such as the feathers we know today, but very short. Some dinosaurs had them perhaps on their heads, and some perhaps on their arms, until through ritualistic selection some female dinosaurs began liking the mutations of longer dinosaur feathers, especially, perhaps, around the arms and on top of the heads of the males.
Time passed by, as better feathers created a more complex display of rituals which demanded more better feathers, which were better looking and better arranged. If we could go back through time I would imagine that the dinosaurs who have these feathers on their arms, cornered their females as they flashed their beautiful colorful feathers at them flapping their arms up, then down again. So the dinosaurs who had the most beautiful feathers in the best displayed order, where the ones that mated.
Time went by and some of these dinosaurs now had feathers all over their arms and tail. These feathers also became of some good use to some dinosaurs who needed to escape their predators, for some of them found out that by putting their arms to the side in an aerodynamic horizontal manner and then flapping them to the back in a vertical manner they could give themselves a little push and give less stress to their running legs while being chased. Also they where probably able to change directions much faster while running by putting the left arm to the side in a vertical manner to make a fast left or the right arm to the side in a vertical manner to make a fast right, which gave them an even bigger edge at escaping their predators.
This was the mayor adaptation that made dinosaurs into flying birds, for the better the designs that these feathers became to help these dinosaurs escape their predators and the more skillful these dinosaurs became at using those feathers, the more these dinosaurs began to leave the ground, until one day some dinosaurs although were not able to fly for long distances, they were able to make high long jumps, such as the way chickens do today, and escape their predators.
After that, some of these dinosaurs became aware that by flying short distances, they were able to jump on top of trees to escape their predators and were also able to get food and seeds which they could not reach before. And this was the mayor behavioral change that made the dinosaurs fly and become the creatures we know today as birds.
This theory still under construction.