Pellet? alot of duckling can't eat pellet when young can you crunch to resemble crumble and try that? How old are these ducklings? Duck do drink alot of water but maybe it's trying to fill up on water since it won't eat the crumble, also you can try adding some warm water to break the pellets down and see if this duckling will eat this way you just don't want to leave wet food out a long time it can mold pretty quick. so try a small amount.
Do you still have them under heat lamp? sometimes they can get over heated and that may cause them to drink more and pant too. another thing to think about if they have been outside running around could the duckling have eaten something it shouldn't have .
How long has the duck not been eating? moldy feed and mold around the pen and coop can also cause problems. Not saying you have these things but you might want to take a close look. At the end of the day any food dropped outside of the feeders here gets scoop up and thrown out if I miss any by next day it has mold growing on it and mold is toxic to poultry. So keep a close eye out and if you suspect toxin then there are ways to treat. Any other symptoms other than not wanting to eat the pellets?
