Feather pulling while swimming

[Mallardmom]

I'm new to the duck world so I'd like someone to tell me if this is normal duck behavior. I have five 6 week old Mallard ducklings. When I turn them loose in the kiddie pool the biggest one will go around pulling feathers out of the other ducks and eats them. Is this normal ? It's upsetting to me

. Also at what age do you call ducklings ducks ???