The optimal temperature for incubating duck eggs is typically around 99.5°F (37.5°C) when using a forced-air incubator. If you are using a still-air incubator, the ideal temperature is slightly higher, around 101°F (38.3°C measured at the top of the eggs).
It's also important to maintain proper humidity levels during incubation. For duck eggs, a humidity level of about 50-55% is recommended for the first 25 days, and then increased to about 65-70% during the last few days prior to hatching. Always monitor the eggs regularly and adjust conditions as needed for the best chances of successful hatching.