A third egg was laid on March 18, 2011 and the parents have now begun pretty much full-on incubation. Both parents will incubate the eggs, but the mom typically does the majority of the work. Incubation lasts between 30-35 days. Note that they continually replenish the fresh greens in the nest. Similar to last year the plastic bag seems to periodically disappear and reappear. When hawks lay large clutches, incubation often really does not begin until the third egg. There may well be one more still on the way.
A few words about incubation: 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!
Thanks to Krista Bradford in Michigan for continuing to provide great screen shots of what is happening on Raptor Cam. The pictures below were all captured by Krista: