Ospreys transfer their heat via a temporarily bare area of abdominal skin called the brood patch.
Large birds of prey such as ospreys and eagles have small brood patches which corresponds to a longer incubation time.
Ospreys can sense their embryo’s movements and temperature with receptors in their brood patches. This helps them regulate their attentiveness (time spent incubating) more accurately. Since the embryo itself increasingly generates heat as it develops, periods of attentiveness can decline as incubation progresses.
“When you see me rocking back and forth on my eggs, I am positioning them into my brood patch.”
The eagle’s brood patch