Hi there, welcome to BackYard Chickens!
Seizures can be caused by any disease condition that causes a sudden electrical discharge to the brain, symptoms will vary with each bird, but some may experience lack of appetite afterwards, disorientation, paralysis of the anterior neck muscles, partial seizures, etc. How is she doing now, is she moving around well, this can help determine the underlying cause of the seizures.
The most common cause of seizures in adult ducks seems to be toxicosis ( Most commonly lead), bacterial/ viral, neurological damage, or a vitamin deficiency specifically in thiamine (B1) or vitamin E.
We'll need some more information about the bird and her past history to help narrow down a possible cause. What are they being fed, have you dealt with any problems in your flock in the past, if so explain, do you have an open or closed flock, is there any chance she may have been in contact with anything toxic, a few common sources of toxicity may be, toxic plants, fishing weights, staples, nails/screws zinc coated, hardware cloth pieces, moldy feed, or bedding, decaying plant matter, or dead animal carcasses, has she experienced any neurological damage in the past?
Could you post a picture/video of her, and her feces? Is she eating/drinking?
She should be placed in a dog kennel located inside the house, in a warm, quiet, stressful free environment, with birds experiencing seizures, it's often recommended that the cage be padded to prevent further damage when experiencing one, if the duck seems restless and depressed the feed and water containers should be shallow enough to prevent drowning or aspiration. Is she blinking ok, if not her eyes will need to be lubricated. Ducks experiencing seizures often become hyperthermic so watch for any signs of overheating.
Seizures in birds is serious, if you have the ability, she should be taken to a vet as soon as possible. They'll be able to offer advanced supportive care, use certain drugs to reduce the frequency of the seizures initially, and use diagnostic testing to help narrow the underlying cause.
