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How to hatch a mallard duck egg
Look for an incubator with a fan.
Using an incubator will ensure the egg stays at the right temperature for hatching. You can get a small incubator for eggs with a fan at your local pet store or online.
The incubator should also come with a wet bulb thermometer to allow you to monitor the relative humidity. You can sometimes find used incubators online for a lower price.