patch of featherless skin on the abdomen that develops in incubating adults; the region sheds its downy feathers (not the overlying contour feathers, whose follicles in Bald Eagles are well to the side); blood vessels increase in size and number (vascularization), greatly enhancing their capacity to produce heat; tissues accumulate fluids, causing the skin to swell and become puffy, exposing more surface area of bare skin to the eggs & young chicks; as parents wiggle into the nest cup their contour feathers spread apart & folds of loose, warm skin envelop the eggs & chicks; sensitized nerves enable the parents to detect the temperature of the eggs so they can modify their posture to optimize heat transfer; hormones (estrogens, progesterone, and prolactin) induce development of the brood patch beginning a few days before ovulation; de-feathering is nearly complete at ovulation, while full vascularization occurs a few days later; sometimes called incubation patch; see Reproduction and Hormones; photo © Thomas Demma