Botulism wouldn't cause death so quickly and you would have noticed progressive neurological symptoms. I had a drake with botulism, and his buddy, who was like his Siamese twin, was unaffected. This definitely sounds like some type of poison since it killed so many birds so fast. A few years ago I had two 4-week old goslings that I was raising with ducklings the same age. I put them all to bed one night and found the goslings dead when I opened the crate the next morning. The ducklings were all fine. The experts I consulted could only agree that it must have been a toxin they ingested, but we had no idea what that might have been and have never figured it out. I'm sorry for your loss and wish you luck in solving the mystery.