Precursor cells that can make a thumb are still there in the bird’s genetic make-up but die off in the egg during development and never produce a thumb in adult birds. Instead the innermost digit of a bird's three-digit wing acts like both a thumb and an index finger.
But what if birds did regress back to having 5 digits? Wonder no more. In a bizarre new advertiser’s world, birds have forsaken their wings for human hands with fingers. Why? So they can text with opposable thumbs instead of pecking out page after page. See video: http://youtu.be/NUGBQkHnWtY