My Khaki Campbell drake has been shaking his head and occasionally making a weird "cough" sound. The weird sound has been going on for a couple of months. The other duck shows no signs at all and they together all the time. I do free range them during the day and they eat whatever they can find besides the flock feed I give them. Does this sound like gape? He is not "gasping" for air and is not limping and appears healthy. Can gape kill? If so, how long? How do I treat?