The embryo must stay warm if it is to develop inside the egg . Because the embryonic chicks do not produce enough metabolic heat during development to sustain themselves, the parent must supply warmth, which it usually does by sitting on the eggs. Most species of bird have a 'brood patch', an area on the breast that loses most of its feathers and becomes suffused with blood vessels during the breeding season. If the egg or eggs are kept in contact with the brood patch, their temperature is raised to near body temperature. In hotter climates, parent birds may not need to spend so much time covering their eggs. The part played by the sexes in incubation differs from species to species, from equal sharing to incubation by one sex only.