Embryos do not produce enough heat to compensate for heat loss from the egg, so the red-tail parents must incubate the eggs until they hatch. During breeding season, females will often develop a "brood patch," an area on the belly which loses its feathers and becomes engorged with blood vessels. This area comes in direct contact with the egg and allows the parent to raise the eggs temperature to almost the same level as its own body.
Many people assume that the biggest risk to the eggs is cooling. However, eggs can often survive temporary cooling. Temporary cooling can slow development leading to a later hatch date but often is not fatal. Last year we successfully hatched three peregrine falcon eggs at Audubon that survived nearly six hours of exposure to rain and 45 degree temperatures after a parent was struck and killed by a vehicle. Overheating is a far greater concern---an increase of only a few degrees above the normal incubation temperature can quickly become fatal to an eggs. Given the dismal weather this week, we don't have much to worry about when it comes to overheating!