Austin and Marianne Hughes have stated that
the fact that birds have about one half the DNA per cell as mammals and
reptiles is because they have adapted. The less unnecessary genetic
material they have to carry around, the more energy efficient their cells
are. Of course, this efficiency is important for flight, as flight
requires great energy expenditure.
Genetic material from chickens and humans was compared. It was found that chickens are missing large portions of their introns ("regions of DNA that don't seem to code for anything"). Their introns were only about half as long as those found in humans.
These findings can't be explained by some "ancient genetic accident". The adaptation only affects noncoding DNA. If this were only an accident, other regions of DNA would have to be affected-- and they are not.
The smaller the noncoding DNA strands, the smaller the cell. And the smaller the cell, the more oxygen absorbing ability it has and the more energy efficient it will be. So the DNA adaptation has made for a more energy efficient bird that uses oxygen to its optimum advantage and thus an easier flight.
(** "Chickens are essentially flightless, but they've only recently evolved from flying jungle fowl, and their DNA has not had enough time to change significantly." **)
Kari Thompson - Article Summary
Austin and Marianne Hughes, Why did the Chicken Lose its DNA?, Discover, v. 17, Feb. 1996, p. 16+