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.

Incubation

Female of Oberhausen turning her eggs

Incubation

For an egg to develop normally, it must be exposed for a considerable length of time to temperatures a few degrees below the normal 104 degrees F (40 degrees C) avian body temperature. Indeed, the ideal incubation temperature for many birds' eggs is about human body temperature, 98.6 degrees F. Almost all birds create the required temperature by sitting on the eggs and incubating them, often transferring heat via a temporarily bare area of abdominal skin called the "brood patch."

On the other hand, the embryo inside the egg is also very sensitive to high temperatures, so that in some situations eggs must be protected from the sun. Peregrines often stand over the nest and shade the eggs when temperatures rise.

Embryos are less sensitive to cold than to heat, particularly before incubation has started. Some eggs have been known to crack by freezing and still hatch successfully. Eggs cool when incubation is interrupted, but this is not usually harmful, and few birds incubate continuously. Instead egg temperature is regulated in response to changes in the temperature of the environment by varying the length of time that a parent bird sits on them or the tightness of the "sit."

Peregrines apparently sense the egg temperature with receptors in the brood patches, which helps them to regulate their attentiveness (time spent incubating) more accurately. Since the embryo itself increasingly generates heat as it develops, periods of attentiveness should generally decline as incubation progresses. Attentiveness is also influenced by the insulating properties of a particular nest.
Eggs are also turned periodically -- from about every eight minutes to once an hour.
The turning presumably helps to warm the eggs more evenly, and to prevent embryonic membranes from sticking to the shell.
Both parents incubate. But there is a large difference in time they spent doing so. Males incubated about 3o% of time, with attentive periods of 2–3 h; female attentive period is about 4 h and she incubates 70% of the time.

Both parents are needed in this incubationproces. Because the Peregrine is a bird of prey, food is not somewhere around the corner to pick up. Hunting is required. Given the fact that only 1 in 4-5 hunt attempts are succesvol in striking prey, it's rather time comsuming. The male therefor is the one who hunts. He is smaller, can move faster and is therefor more succesful in hunting than the larger female is. He has proven her during courtship that she can fully rely on him during incubation and later on when the eyas are born, she knows he can provide. That's why she has choosen him, not because of his beautiful brown eyes !

So he is the one who hunts and takes care of the groceries. It's therefore disastrous when anything happens to him during the incubation. She will not be, or hardly be able to incubate and takes care of food at the same time.

Food is not eaten in the nest, but outside. He flies by with food and she leaves the eggs. He takes over incubation, while she eats. And.... of course goes to the bathroom. Afters hours of incubation she really has to go. She can not leave her droppings close to the eggs, because of the infectionrisk. Sanitation is essential for the health of her unborn children. And so she has to sit on a full blatter until he appears.