Looks like her legs aren't working? Could be Mareks, heat exhaustion, or botulism to name a few things that come to mind from that symptom alone.
Many things will cause them to become weak, so it's near impossible to figure it out from that. Best to find a vet to have a look at it, and potentially get an autopsy on the roosters. When one bird is sick I don't worry too much, but when multiple birds die or are sick I become a bit more concerned.