We just hatched some Muscovy ducks and I kept the humidity around 55-60% and the temp around 99-100 during the incubation period. Then at lockdown I bumped the humidity up to at least 75% and kept the temp around 100. Out of the 7 that made it to lockdown, 5 hatched, 1 had stopped developing along the line, and 1 had internally pipped but died for some reason.
I rarely manage to get my humidity above 60% even at hatch, I find my eggs don't hatch as well as I'd like, I'd love to be able to get it higher!
There is often a great deal of debate on this topic, and the bottom line is that ideal humidity for you may range from low teens to high 70s, depending on several factors (your specific incubator, the ambient humidity, and altitude being the prime concerns). Finding your ideal humidity is a matter of practice and fine-tuning. Generally, I recommend running the first hatch at the same humidity recommended in the owner's manual that came with it, even if it says that's the recommendation for chickens. It's usually a good starting place. During that first hatch, take good notes. In particular, make a note of the size of the air cell at each one-week candling. Ideally, the air cell should be approximately one-third the volume of the egg by lockdown. Anything between 1/4 and just over a third is fine. Less than that, and the babies will probably drown when they try to pip internally; more than that and they won't have room to grow. If you end up with air cell that is too large, run your humidity higher next time, and take notes again. If the air cell is too small, run it lower next time. I like to actually draw a circle on the eggs in pencil at each candling, where the air cell is. This has the additional benefit of showing me where to expect the pip to occur. Usually, the incubator's recommendations are close enough to give you a decent hatch. However, if you live in a very dry climate you should probably raise the humidity for good measure. Likewise, if you live in the SE U.S. where the air feels like a wet blanket in summer, you may want to run an entirely dry hatch (which is what I do with great success in summer). After lockdown, the rules change. I have found that the higher the humidity the better, but I don't bother measuring it any more. Basically, if the inside of the windows has condensation on it, you're good. If not, raise the humidity. Good luck!
