Note that in each stage, the current configuration has already set the stage for natural selection to "prefer" individuals which better meet the requirements of the next stage.
Why can't each "stage" of the bird shown, simply be similar birds with slight differences, such as the differences between a grackle and a crow?
That's actually an excellent question, and I'm glad you asked it.
...I'm just not glad you asked it right *now*, because I need to get to bed and a proper anwer will take at least 30 minutes which I don't have at the moment. Can I give you a raincheck for now, and I'll get back to you on it later today?