Shining Sunbeam

The shining sunbeam is 12 to 13 cm (4.7 to 5.1 in) long. Males weigh 7.6 to 8.1 g (0.27 to 0.29 oz) and females 6.9 to 7.5 g (0.24 to 0.26 oz). Both sexes have a short, straight, black bill. Unique among sunbeams, neither sex has any white plumage. Males of the nominate subspecies have a dark brown crown and upper back; their lower back and rump are lilac-gold becoming silvery green on the uppertail coverts. Their tail feathers are dark brown to rufous, usually with bronzy edges. Their throat is rufous dotted with dusky gray, the breast a patchy buff, and the belly reddish brown. From north to south, the male's underparts become brighter, the crown darker, and the tail redder. Females are similar to the males but the gold and green of the lower back and rump are duller or absent. A. c. caumatonota differs from the nominate with an iridescent amethyst rump and purplish amethyst uppertail coverts.

The nominate subspecies of shining sunbeam is found in all three Andean ranges of Colombia and south from there through Ecuador to Peru's departments of La Libertad and Huánuco. A. c. caumatonota is found in Peru from the nominate's range south to the departments of Lima and Cuzco. The species inhabits sub-páramo to páramo, landscapes characterized by grasslands with scattered shrubs and trees. It also inhabits cloudforest and semi-arid ridges with a few trees. In elevation it ranges from 2,500 to 4,300 m (8,200 to 14,100 ft).

The shining sunbeam forages for nectar at all levels from the ground to treetops. It feeds at a wide variety of bromeliads, vines, and trees including Embothrium, Puya, and Fuchsia, and males defend patches of flowering plants. The species also feeds on small arthropods by hawking.