>>I appriciate your illustration, but both the best and worst sighted birds remain birds throughout this whole process.
In the very last iteration of change (getting slightly better sight), that's true. But what if the poor-sighted population finds that life on the ground is better for them? They can't spot their food before the better-sighted ones do, so they just stay on the ground and grab whatever's handy. They would all be one type of bird at first, just some going to the ground more often. But those on the ground would mate with each other, and those in the trees would mate with each other. More and more, the ground dwellers would stay there. They would develop stronger legs, more vicious beaks (for defense), and be more aggressive. (Maybe).
In time, these would become two different species. Granted, they're still birds. But they've changed. How many more changes would it take before the bird wasn't a bird in any accepted sense?
In "getting" evolution, the easy part is seeing the small changes, like eye-sight. The hard part is understanding that these small changes can accumulate until the thing you end up with is almost nothing like the thing you started with. Genetics may end up the only clue to tie one to the other.
Natural selection is driven by the environment. Split a population over two different environments and they can't help but adapt to them. Split again, and again... the changes add up.
Dave