Thank you guys. I did not start the niacin until today after I panicked and searched the web.

Stupid newbie duck mom move.
Unfortunately, around 5pm when my husband got home, he found her laying in the cage, convulsing, with foam coming out of her mouth.
The area that they are in is completely bird friendly. My chickens are my pride and joy and I am super on top of keeping everything safe. The ducks now share the area and they all get along really well. I don't know what happened. She is the only one that has acted this way and I'm crossing my fingers that it began and ended with her.
It's probably unrelated, but she is also the duck that had the outward growing wing feathers as well. All the others have developed normally.
Thank you again for the help and advice.
Sara