Abrood patch, also known as anincubation patch in older literature,[1] is a patch of featherless skin on the underside ofbirds during thenesting season. Feathers act as inherent insulators and prevent efficient incubation, to which brood patches are the solution. This patch of skin is well supplied with blood vessels at the surface, enabling heat transfer to the eggs when incubating.[2][3]
The positions of brood patches can vary. Many have a single brood patch in the middle of the belly, while someshorebirds have one patch on each side of the belly.Great Auks andRazorbills develop two brood patches on the side of the body.[4]Gulls andGalliformes may have three brood patches.[citation needed]American kestrels develop three brood patches.[5]
Pelicans,boobies, andgannets do not develop brood patches but cradle the eggs on their feet.[3]Brood parasiticcuckoos do not develop brood patches.[6] In species where both parents incubate, brood patches may develop in both sexes,[3] though the size of the patch may vary by sex and size of the bird.[5]
Shortly before egg laying, hormones such asoestrogen,prolactin, andprogesterone cause a bird’s belly feathers to loosen and fall out, thus forming a patch of bare skin.[7] In most species, the feathers in the region shed automatically, butducks andgeese may pluck and use their feathers to line the nest. Feathers regrow sooner afterhatching inprecocial birds than for those that havealtricial young.[3]
Brood patches help carry heat close to the skin surface during incubation. Additionally, birds may accumulate fat around the patch to increase its pliability.[7] Upon settling on a nest, birds will shift in a characteristic side to side manner to ensure full contact of the brood patch with eggs or young.[3]
Brood patch can help regulate number of eggs produced in one cycle, orclutch size. Once eggs are laid, the touch sensors in the brood patch detects the eggs and trigger hormonal responses to limit ova production. However, if the eggs are removed or broken shortly after the eggs are laid, this hormonal regulation will not happen, and eggs can be continuously produced.[7]