Pecking Duckling

[ChickenNewbie5]

We have (3) 4 week old ducklings. Not sure of gender yet as the feathers are in on some parts but not all the way. *I* think it might be 2 girls one boy. Anyway, in the last 4 days, one duckling is pecking at the other ducks. I've had to remove it from the other ones. What is causing this and could it be diet related?
They are eating Duck Mash 18% protein. No treats yet...

 

[Amiga]

I have read that giving them fresh greens daily can reduce pecking behavior. Also, crowding can cause them to become more aggressive with each other (I don't know what your setup is).

Do they get outside, or someplace they can run around a bit?