One of the posterior wings of an insect.

Any one of numerous species of noctuid moths belonging to Catocala and allied genera, in which the hind wings are banded with red and black or other conspicuous colors. Many of the species are called red underwing.

moth having dull forewings and brightly colored hind wings

The underwing is the bottom side of the wing. When the bird is stationary, it is the side pressed against the body and when the bird is in flight, it is the side one can see standing on the ground looking up at the bird.

Underside of wing.