I’d try during the day. My mauve Orpington was sitting on non fertile eggs so while she was out of the nest to eat I took all the eggs and put the ducklings in her nest when she came back in she looked at those chicks stepped into the nest covered them and boy was that a sweet sight. Only problem I see with doing it at night is you don’t stick around to see if she accepts them they may not make it. But this is just my experience others have probably done great with putting them under at night.