Duckling aggression

[Kiwiduck34]

I’ve got 4 mallard ducklings and they are 3 weeks old now. One of them recently has started biting the other three. She bites them almost every time they get near her and the others always scream and run away. It’s gotten to the point where the three non-aggressive ones don’t want to be in the kiddie pool when she is and today one of them had a bloody pin feather, I assume from being bit. All four of them have been together since they hatched and they aren’t aggressive towards me. Has anyone ever seen this behavior before?

 

[oldhenlikesdogs]

How big of an area are they in? I've seen some nibbling on siblings in my muscovy ducklings, enough to make the pin feathers bleed. Mine grew out of it. What are you feeding? Sometimes too much protein can make birds squirrelly.