A birding expert corrected my interpretation of the Molting Male Eastern Bluebird. They explained that what I am seeing is a brood patch. See the explanation below.
From Wikipedia 20250428: The brood patch is a patch of featherless skin on the underside of birds 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.
Earthwatcher 