Question: Why is this peregrine falcon lying face down on her nest?
Answer: She's sunbathing.
Many birds sunbathe including robins and mourning doves. There are various theories as to why they do it - perhaps to eradicate parasites, perhaps to straighten their feathers, perhaps to spread the preening oil in their feathers.
They also might do it because it feels good. “Ahhhh.”
See Chuck Tague's blog here for more information on sunbathing (even swallows sunbathe).
(photo from the National Aviary webcam at the University of Pittsburgh)