Male adults have black scapular feathers with a greenish tone, as well as the upper parts and rectrices along with the uppertail coverts. The greater coverts are white or light gray.
On the other hand, adult females have brownish scapular feathers, upper parts, and rectrices. Greater coverts are uniform in color, light yellowish-brown, or light brown-gray.
It is possible to recognize 3 different ages:
Juvenile specimens: with a design similar to that of the adult female, but with the feathers of the forehead and crown with wide light brown edges. Clear difference in the dark-striped neck, the scapular feathers and upper parts are dark brown with wide light brown edges. Lesser and median coverts of the wing are dark marres with wide light brown edges. The primaries with a sharp tip (“sharp”).
2nd year specimen: they have the retained wing coverts of the juvenile plumage, they look darker than the molted adult design.
Adult specimen: they have darker inner secondaries, all with a similar design. The median and greater coverts are white or gray. In these specimens, the primaries are somewhat rounder at the tip.
The Little Bitterns perform a complete post-nuptial molt that starts in the month of July (suspended before migration and finished in the wintering areas). The post-juvenile molt is a partial molt of the body feathers and some inner secondaries changed after arriving in the wintering areas. In both ages, a partial pre-nuptial molt takes place that includes a variable number of inner secondaries, back, and scapulas.
Primaries: 10 per wing.
Secundaries: 13-15 per wing.