Hi, there, welcome to BackYard Chickens. So sorry to hear about your duckling, we'll try out best to help, but we'll need some more information.
What are they being fed? What is the expiration date on the bag? What kind of water, and feed dish is being used? What are they being kept in? Where did you acquire the ducklings? Is she eating and drinking? Could you post a few pictures of her poop?
Seizures, convulsions, and spasms in ducklings can be caused by a numerous amount of things, ranging from ingesting toxins, vitamin E or Thiamine deficiency, trauma to the head or neck area, neoplasms, metabolic diseases, and viral or bacterial diseases.
Ideally, and what I would do here, is start off by physically examing the duckling, from head to toe for anything alarming, then taking an anamnesis of the bird, and use that information to understand what condition would be more likely, e.g. "are more then one bird affected" if only one bird is affected this would make it less likely for it to be a viral or toxin-related condition.
I've had birds like this as well, and after I've done a physical examination, and past history check, I start off by checking their blood glucose levels using a blood meter to test if the bird is lacking sugars (this is often a prevalent problem with shipped ducklings (Hypoglycemia). I'd smear their blood on a slide, and stain it with a Romanowsky type stain, and look at their WBC and RBC morphology for any abnormalities, that might point towards toxicosis. I would additionally take some cultures from their feces and mouth to check and see if there might be a bacterial infection going on (premature ducklings where their abdominal area is still exposed can develop this problem). Of course, you probably don't have the supplies to do these diagnostic tests so you will have to treat by elimination.
If I were to suggest anything, I'd first suggest that you start this duckling on vitamins, specifically thiamine and vitamin E, for thiamine you can drip a few drops of Cattle B complex along her bill, and for the vitamin E you can give her one 400iu soft gel once daily. These nutritional problems (wry neck) are common in ducklings, and we can start by off by eliminating one of the more common causes of seizures. If we don't see any improvement, we can consider something else.
Additionally, it’d be a good idea, to offer her scrambled eggs which will boost vitamin E absorption, and provide her with other needed minerals. Keeping her warm, and in a stressful free area is also important to her health.
