Feather pulling and eating..

[MarieNC]

Hi all - I have two Pekin ducklings, about 5 weeks old. Not sure of the sex of them yet but I suspect, by their vocalizations, that both are male. They are very sweet and friendly to me. One is just slightly bigger than the other one, and the bigger one frequently pulls out the feathers of the little one and eats them, particularly when they're swimming together. I know this can be a diet deficiency in chickens (I have six of those) and wondering if I should be supplementing them with something besides the Purina Flock Raiser they're now getting. Wondering if they're getting too big for their space but let them outside in the run with my other two ducks (two Rouens) and there was plenty of leg room, yet the picking continued. Anyone have any advice? Thanks in advance.