Help!!! 2 ducks died

[Ann231]

Last month one of my mallard drakes died suddenly. Then my pekin hen has been lethargic and kinda using her wings as balance to walk. She wasn't eating much & I think my other ducks & chickens were picking on her because she was missing feathers. This had been going on for a week or so. She seemed to get better, I separated her and gave her food & water. Yesterday she was looking bad again & this morning she was dead.

I'm new to birds so I really have no idea what's going on or if it's the same thing with each duck.