Debbie Fulton from the Fort St. Vrain Eagle fan group got this excellent capture of a brood patch while Mom did a little sunbathing yesterday! Feathers are great insulators, but unsuitable for transferring heat. Shortly before eagles lay eggs, the hormone oestrogen plus a secondary hormone (prolactin or progesterone) causes feathers on birds' bellies to loosen and drop off, creating a patch of bare skin. Oestrogen also controls the development of supplemental blood vessels that bring warm blood close to the surface of the skin, further aiding heat transfer. The brood patch helps eagles incubate eggs even in the coldest weather (a memorable -50F/-45C when egg #2 was laid in Decorah in 2014)!In precocial birds, feathers begin growing back as soon as the eggs hatch. In altricial birds (including bald eagles), patches remain functional through early brooding (my guess would be 15-20 days, or about the time some of Mom's lethargy starts to fade). Then they gradually disappear, restoring the area to non-breeding function and feather cover about the time the young are fledged.
Why do eagles sunbathe? We believe it helps kill and/or prevent parasites in less-exposed areas like wingpits, and it also looks quite comfortable. Thanks for the great capture!
Links
- Brood patches: https://web.stanford.edu/group/stanfordbirds/text/essays/Brood_Patches.html
- Hormonal control of brood patch development: http://joe.endocrinology-journals.org/content/26/1/11
