I don't think its greed or malice that drives them, but simply not liking other things in the nest. They grow out of trying to expel their siblings when they're unsuccessful.
The cuckoo chick is not evil. It cannot see or think. It has a 50% chance of surviving which is less than most birds. Its born with a sensitive and muscular back. When anything touches its back, it simply feels a instinct to push back hard until theres nothing there. Is it simply driven by an "annoying" feeling like scratching an itch? Or is it a feeling of terror/doom like when a baby monkey has nothing to cling to? We cant know. But we know a newborn chick has no g.d. idea what it is doing or why.
mother bird: wait a minute this one is a cuckoo!!! Ill never feed that ever again you'll starve to death! Baby cuckoo : WAIT HOW DID YOU KNOW!? part 2 tomorrow
