What is a brood patch?

Daylight length, or photoperiod, strongly influences hormone production in birds. In the northern hemisphere, our story begins shortly after the winter solstice in December. As daylight length increases, a cascade of hormones causes birds’ gonads to swell in preparation for reproduction, egg-laying, and incubation. In this blog, we’ll discuss the role the brood patch plays in incubation and determining clutch size.

How do bald eagles keep their eggs warm in subzero temperatures? They apply heat via a special area of bare skin on their bellies called a brood patch. Shortly before laying, the hormones oestrogen, prolactin, and progesterone cause feathers on a bird’s belly to loosen and drop off, creating a patch of bare skin.

Oestrogen develops supplemental blood vessels in and around the patch, carrying warm blood close to the skin’s surface. It may also cause eagles to store fat in and around the patch, making it swell and increasing its pliability.  A brood patch allows eagles to incubate through the very coldest weather: a record -50F/-45C when Mom laid her second egg in 2014!

Brood patches help determine clutch size

In addition to applying heat, a female eagle’s brood patch helps to regulate the number of eggs she lays. Bald eagles are indeterminate layers: although they lay only one clutch per year under normal circumstances, they will lay new eggs to replace eggs that are removed or broken early in the laying cycle. How does it work? When touch sensors in a bird’s brood patch detect eggs, they initiate a complex cascade of hormones that limits further ova production. But if a bird’s eggs are removed or broken shortly after she lays them, regulation doesn’t happen. She will continue laying until she can’t produce any more eggs or lays enough eggs to trigger ova regulation. In one study, a house sparrow laid 50 eggs before finally stopping!

Does diet help determine how many eggs an eagle lays? Female bald eagles store water and shift fat to their brood patches in preparation for egg-laying and incubation.  Since brood patches help regulate egg-laying, it seems plausible that patch size could influence clutch size. All of the eagles we watch live on territories with an abundance of food, which may explain why they consistently lay three eggs instead of two.

Do bald eagles have brood patches all year long? They do not! Egg production hormones ebb when hatch begins. Feathers grow back, blood vessels shrink and disappear, and stored fat dissipates, restoring the area to non-breeding function and feather cover around the time young eagles fledge.

Things that helped me learn about this topic

  • The book ‘Bird Sense’ by Tim Birkhead. The book is divided into chapters seeing, hearing, touch, taste, smell, magnetic sensing, and emotions. It’s a fascinating read and I highly recommend it.
  • The book ‘The Bald Eagle’ by Mark Stalmaster

Why do eagles sunbathe? It feels good and helps kill and/or prevent parasites in less-exposed areas like wingpits, helps keep feathers in tip-top condition, and looks quite comfortable, too. Thanks for the great captures, Robin and Debbie!