Better tell that to the bats, bees, honey gliders, flying squirrels, flying fish, pteradactyls, pteranodons, etc., etc.
Irrelevant. We're talking about evolving into a flying bird and a gliding squirrel or lizard is not remotely close to a plausible 90% point for getting to flying bird status. For a bird or anything trying to evolve into a bird to fly, even badly, all features would need to be in place.
You could "fly" yourself if you wanted to. Especially if you first climbed a tall tree and relied on gravity for acceleration. At least you could accomplish a steep dive without any wings, feathers, hollow bones, enlarged respiratory system, or big breast muscles. Because you don't know what the first proto-bird looked like it is a bit presumptuous of you to assume that it could not do at least as well as you as a "flyer" of sorts. This proto-bird may have been very well adapted for steep dives, perhaps it hunted this way. Perhaps it had sufficient control surfaces to adjust its trajectory while diving. It really doesn't take very much to begin to fly.
Now to your second mistake. No living thing "tries" to evolve. Becoming a bird is what happened to the progeny of those collections of breeding individuals for whom survival depended on falling without damage which led to diving with accuracy which led to gliding for distance which led to soaring aloft which led at last to flying. The best at each stage tend to leave more progeny than the less accomplished and the process accelerates when a species moves to an unoccupied niche (i.e., the sky is the limit).