Are they too cramped, I wonder? Do they have 2 or 3 square feet each in the brooder, with time to stretch their little legs during the day in the tub or larger safe area?
I have read that ducklings can pick at each other if they are kept in too small a brooder.
My runners outgrew their brooder in less than two weeks.
And I need to ask - is the heat lamp close enough for them to touch it? If so, you need to move it up and back.