Safe environment — Place the animal(s) in a small covered cardboard box or plastic carrier with plenty of air holes for ventilation. Use soft bedding such as towels or old t-shirts. Do not use grass clippings, wood chips/shaving, newspaper or paper towels; these types of bedding become damp very quickly which can lead to hypothermia. Keep the box inside in a quiet, dark area away from pets and people. Handle the animal as little as possible; even young wild animals get stressed by captivity and over handling. Change the bedding if it gets wet. If the duckling(s) are jumping a lot put them in a shallower box (such as a shoe box) with lid so they cannot jump themselves to exhaustion; do not use a pet carrier or aquarium for these types of ducklings.
Provide a heat source — Infant birds cannot regulate their own body temperatures adequately and are susceptible to hypothermia. You can place a heating pad set to LOW under half of the box. If you do not have a heating pad you can fill a bottle with hot water, cover it with a barrier (towel) and place it inside the box. Water bottles will need to be checked frequently for loss of heat over time. It is important for the animal to be able to move close to or away from the heat source.
A very shallow dish (peanut butter lid, etc.) of water can be placed into enclosure for drinking purposes. Do NOT let them swim or get wet! They are reliant on their mothers for waterproofing and heat and can die from hypothermia if allowed to swim.
Commercially available duck/goose starter can be offered for food in a shallow dish. Other temporary alternatives for overnight only are crushed up Cheerios, bland hard cereal or cornmeal. Do NOT give bread, seeds/corn, chips, popcorn, etc. If fed inappropriate things young waterfowl are susceptible to getting an intestinal blockage.
Get the animal to a licensed wildlife rehabilitator or facility as soon as possible in the morning. It is against the law and not in the animal’s best interest to keep or raise wildlife without the proper permits and education.
Always wash your hands thoroughly after handling a wild animal.