I havnt actually had a chance to hatch any call eggs yet, but I have had pacific blacks for years and they are very like calls. When I was incubating my pacific black eggs years ago and following incubating and candling instructions from sites on major breeds, it didnt seem to quite match up. When I found a page on calls, suddenly everything was making sense.
Smaller birds, smaller eggs. Seem to prefer cooler temp and longer gestation as opposed to large breeds like pekins that prefer warmer temps for the eggs. Lots of humidity too. It seems with pekins and muscovys you follow the general instructions and get a good 60 to 80% yield. Calls, like pacific blacks, just dont roll like that! If you have 10 eggs, each individual developing duckling has its own idea of what it wants....So I shoot for a middle ground to hopefully get the best yield.
As I said I havnt had a chance to incubate my call eggs yet, but with pacific blacks I was over the moon to get one duckling from 5 eggs! It is challenging!