Whether darting from flower to flower, or floating in place over his territory, the male Peruvian Sheartail is a distinctive sight. White patches in its elongated tail-streamers catch the eye as they flutter and whip behind the bird, giving its flight a sprightly, airy quality. Even more striking is the way the male holds these inner tail feathers stiffly out to the sides when courting a female. The entire tail is split, with half projecting out to either side, during display. Courtship is a frenetic affair, with the male flying rapidly back and forth in front of the female, glittering gorget flared, squeaking stridently, and making a thin whining sound with the outer tail-feathers. As in many hummingbirds, females are less brilliant, with a green back and pale underside.
Peruvian Sheartail is a desert species, living in arid coastal Peru, from near sea level up to ca 2400 m. Recently it has expanded its range south into the Atacama desert of Chile. It was first reported in Chile in 1971 (Johnson 1972); after a second report in Azapa Valley in 1992 (Howell and Webb 1995) its population rapidly grew and it is now more common than Chilean Woodstar Eulidia yarrellii (Estades et al. 2007). Presently the sheartail is common in Azapa, and a small (but likely growing) population is present in Vitor Valley. At the same time the sheartail has expanded its range, the endangered Chilean Woodstar has experienced a population crash; competition with the Peruvian Sheartail is one possible cause of this decline.
Thaumastura cora was described as Ornismya cora by René Primevère Lesson and Prosper Garnot (1827), naturalists aboard the Corvette la Coquille, who encountered it between Callao and Lima. Cora was the name of a fictional Incan princess, the heroine of Marmontel’s play ‘Les Incas’ (1777). Bonaparte (1850) placed it in the monotypic genus Thaumastura, which means "wonderful tail". The sheartail is closely related to Short-tailed Woodstar (Myrmia micrura), Purple-collared Woodstar (Myrtis fanny), and Oasis Hummingbird (Rhodopis vesper).
Peruvian Sheartail is locally common but has a somewhat small range. As a result, relatively little is known about its natural history.