According to the Birder’s Handbook, in most bird species, the parents share incubation. In monogomous species in which both adults incubate the same clutch, like the American Bald Eagle, the eggs are covered most of the time.
Eagles must keep the developing embryos inside the eggs warm for a long period. This incubation process takes, on the average, thirty-five days, but plus or minus a few days. For many birds, if the temperature of the eggs increases, it shortens the incubation period.
According to Carolina Raptor Center records — where specific, Savannah and Derek average about 33 days in incubation.
Eagle information from The Bald Eagle by Mark Stalmaster.