Yeah I keep my young ducklings away from my chickens too.
Very sorry you lost this little one. Usually if a chickens gets one there are injuries especially at the eyes.
Keep us posted looking forward to seeing some ducklings and mama. Make sure to keep and ducklings safe from the others if mama keeps sitting while hatching is going on she isn't going to be going after a stray duckling which means others can do it harm.
Sometimes it works with 2 females being broody together but it had never here. The one who isn’t on the nest will most likely not accept ducklings give her time she’ll get over it. It takes a while for the hormones to calm. Keep us updated on the last 3 eggs. Make sure when hatch begins a...
Where was the mom? who else is in with her? sometimes new moms just don't know what to do with a duckling that just hatched. Was it wounded? What breeds?