I've had a flock of ducks now 4 or 5 years. They have a spring fed creek and other than predators they have always been very healthy. I noticed a neighbor being aggressive towards them a couple times so when one died for no apparent reason I suspected she was poisoned. Another neighbor had seen her fall over and die on the spot with no apparent injuries. She had disposed of the duck before I got home. Today I noticed my older Pekin "Quacky duck" couldn't quack. Her mouth was opening and closing but no sounds. She's sitting and her tail is bobbing up and down like she's having trouble breathing. She is eating and drinking but something is wrong. Is there any other reason why this would happen? They are not penned and live in a very clean environment.