Almost all birds incubate their eggs: keeping them warm while the embryo develops into a chick. In order to transfer heat better from their body to the eggs, many birds develop brood patches (a.k.a. incubation patches). The bird loses feathers from her belly, and the bare skin becomes wrinkly and swollen with fluid. In juncos only the female develops a brood patch, since she does all the incubating, but in species where males also incubate, males can develop brood patches too.