Make sure it has a heat source to stay warm. A 40-60 watt light bulb is probably all it needs. Keep it about 2 feet above the sleeping area, but allow some other area to be without direct heat from this bulb, so the duckling can find a comfort zone. You should have the temp under the lamp at...
I don't think you gain anything by giving it back to mom. I think the mother thinks its time to ween it off. The only problem is that it still needs some warmth. It can get chilled and die. I would bring it in and keep it in. Don't worry about it being touched by humans. Birds cannot smell...