Each egg is rolled about once an hour to prevent the embryo inside from sticking to the eggshell. The rolling also ensures that all parts of the egg get heat from the parent’s brood patch.
The brood patch (see photo) is a featherless area on a bird’s breast that is infused with blood vessels and is sensitive to temperature, and a bird places the patch against its eggs to keep them warm. It’s helpful to think of the brood patch as a hot-water bottle resting against the eggs. The brood patch only appears during nesting season, so when an injured bird is found with a brood patch, rehabilitators know then that the bird has young in a nest somewhere.
http://friendsofblackwater.org/wordpress/eagle11/2011/01/23/egg-rolling-and-eagles/