Broodpatch

When incubation starts both peregrines take turns breeding.
The female peregrine, as well as the male develop a brood patch during the breeding season. Changes in hormone levels like prolactine, progesteron and estradiol during the nesting season start the process. These hormones start ro rise as soon as the second last egg has been layed. Down feathers on the bird's tummy, and even some contour feathers, suddenly get very loose. In the peregrine falcon those feathers just fall out. When the feathers fall out, tissue changes start to happen too. The tissue in the tummy area swells. The tissues hold more water, and more bloodvessels are starting to grow in that area. These changes make the bird's tummy skin almost as hot as the body's interior.
When the incubating peregrine sits on the eggs, the skin muscles open up the brood patch. Then the parent sits down and wiggles its body back and forth a bit. The sensitive skin feels for the eggs so the parent can settle where the skin makes best contact with the eggs. The parent keeps turning the eggs almost every hour. This is done to prevent the fast growing enebryo to stick to the shell and to take care the eggs are evenly warmed by the broodpatch!
For the parent peregrine the area of the broodpatch is a problem in their own thermoregulationsystem of the body. In order to keep it's own temperature on 104F they have to eat a lot, so there is a constant suply of energy.