Brood patch

A bird on its back, revealing a patch of featherless skin
Brood patch of a sand martin

A brood patch, also known as an incubation patch in older literature,[1] is an area of featherless skin on the underside of birds that exists during the nesting 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]

Location

The positions of brood patches can vary. Many have a single brood patch in the middle of the belly, while some shorebirds have one patch on each side of the belly. Great Auks and Razorbills develop two brood patches on the side of the body.[4] Gulls and Galliformes may have three brood patches.[citation needed] American kestrels develop three brood patches.[5]

Pelicans, boobies, and gannets do not develop brood patches but cradle the eggs on their feet.[3] Brood parasitic cuckoos 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]

Formation

Shortly before egg laying, hormones such as oestrogen, prolactin, and progesterone 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, but ducks and geese may pluck and use their feathers to line the nest. Feathers regrow sooner after hatching in precocial birds than for those that have altricial young.[3]

Function

Incubation

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]

Clutch size

Brood patch can help regulate number of eggs produced in one cycle, or clutch size. Once eggs are laid, the touch sensors in the brood patch detect 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]

See also

References

  1. ^ Jones, Richard E. (1971). "The incubation patch of birds". Biological Reviews. 46 (3): 315–339. doi:10.1111/j.1469-185X.1971.tb01048.x. ISSN 1464-7931.
  2. ^ Turner, J. Scott (1997). "On the Thermal Capacity of a Bird's Egg Warmed by a Brood Patch" (PDF). Physiological Zoology. 70 (4): 470–80. doi:10.1086/515854. PMID 9237308. S2CID 26584982. Archived from the original (PDF) on 2022-10-20. Retrieved 2020-07-23 – via EBSCO.
  3. ^ a b c d e Ehrlich, Paul (1988). "Brood Patches". web.stanford.edu. Archived from the original on 2015-05-12. Retrieved 2020-08-03.
  4. ^ Birkhead, Tim (14 October 2021). "The Great Auk had two brood patches". British Ornithologists' Union. Retrieved 31 January 2026.
  5. ^ a b Wiebe, Karen L.; Bortolotti, Gary R. (July 1993). "Brood Patches of American Kestrels: An Ecological and Evolutionary Perspective". Ornis Scandinavica. 24 (3): 197. doi:10.2307/3676735. Retrieved 1 February 2026.
  6. ^ Payne, R. B. 2005. The Cuckoos. Oxford University Press. p. 128.
  7. ^ a b c "What is a brood patch?". Raptor Resource Project. 3 February 2025. Retrieved 31 January 2026.

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