大多数鸟在孵卵期间腹部羽毛脱落形成孵卵斑。孵卵斑是由于真皮正加厚，血管增多造成的，有利于腹部热量的传导。不同类群的孵卵斑数目在1-3个不等，而同种鸟的孵卵斑数是一致的，因此可以作为判断亲缘关系的依据。

A brood patch is a patch of featherless skin that is visible on the underside of birds during the nesting season. This patch of skin is well supplied with blood vessels at the surface making it possible for the birds to transfer heat to their eggs when incubating. In most species the feathers in the region are shed automatically but ducks and geese may pluck the feathers and use them to line the nest. The feathers of the region regrow soon after the eggs hatch in the case of precocial birds but may be delayed in those birds having altricial young.