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), tissues accumulate fluids, and blood vessels swell in size and number (vascularization); parents wiggle to spread the contour feathers apart to expose the warm bare skin to the eggs and young chicks; the patch transfers heat to the eggs, and 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