I agree with Cynthia! Take your latest dead bird and have it necropsied. That's the only way to tell for sure. I'm just wondering...if they haven't been let out for weeks, how much space do they have? Could be in crowded conditions for long periods of time cause death by stress? I don't know. I'm just asking. Also, you said they are given fresh water from the same source three or four times a day. They do have water available at all times, right? Have you listened to their chests to see if there's any wheezing? I hope you solve the mystery soon. I know how frustrated and sad this must make you feel!!!