Not based on any real example, just me ranting
The beak gets shorter. Okay, what about the wings? Say smaller wings allowed for better survival? Great. Oh wait, now predators are in the sky, okay, so the wings are useless because when we fly, the other animals get us...wonderful...so, we stay lower in the ground. Oh wait, trees. Okay, we need better feet to grip onto the trees to help us survive. Oh wait, now out blue colour makes us stand out more, well that's mutated and changed. Wonderful. Hey, wait a minute, are we even the same species anymore? Oh !@#$%, now the predators are lower down! AHH! RUN! Okay, now we need something to defend ourselves with. Wow, look how sharp my talons are! ACK DIE EVIL PREDATORS! Oh no, now the predators all have thicker skin, out talons won't pierce them! Oh no! THE SLOWER, LARGER BIRDS ARE DYING! The small are surviving, YAY!
So, from a Small taloned, large, blue-feathered, large beaked bird. We end up with a small beaked, small, brown feathered bird with large talons. Are they even the same animal anymore? Wait, what happens if they spend so much time on the ground, some of them are slowly born with smaller and smaller wings and manage to survive? Then the wings are mutated away, They clearly are not birds anymore, they lack wings.
Essentially the concept behind evolution is that an animal, which will randomly mutate and change anyway, is also trying to compete with the environment, and other animals, as well as predators. This limits the mutations that are able to survive. Strickly speaking, Evolution is more like a mixture between random mutations, and natural selection. Both of which have been observed on smaller scales in human lifetimes.
Then we get into the rarely bizzare mutations, where chromosomes are lost and gained. That's where things start to get interesting.
