How the Brood Patch Develops
Most female birds, and some males, develop a brood patch during the breeding season. Changes in hormone levels during the nesting season start the process. Down feathers on the bird's tummy, and even some contour feathers, suddenly get very loose. In some species, those feathers just fall out. In other species the mother pulls them out.When the feathers fall out, other changes happen too. The tissue in the tummy area swells. The tissues hold more water, and the blood vessels that feed the skin expand. These changes make the bird's tummy skin almost as hot as the body's interior.
In species where only the female incubates, the male doesn’t develop a brood patch. In species that share incubation like the bald eagle both male and female have brood patches.
The rocking or wiggling of the owl as she begins to lay on her eggs opens the feathers that have curled over the brood patch and puts their skin in contact with the eggs or chicks.
At the end of the nesting season the blood vessels recede and the feathers grow back to keep the adult warm.