Oasis Hummingbird

The oasis hummingbird is 11 to 13.5 cm (4.3 to 5.3 in) long and weighs about 4 g (0.14 oz). Both sexes have a long, black, decurved bill. The nominate subspecies is larger and has a longer and stouter bill than the other two, but the three are otherwise alike. Both sexes have olive green upperparts with a cinnamon rump and uppertail coverts. Females have a small white dot behind the eye and males a longer white stripe. Males have an iridescent gorget that ranges from rosy violet to purple with turquoise to the rear. Their underparts are dull white with a dull green wash on the flanks. The tail is deeply forked; its central feathers are grayish olive and the rest purplish brown. Females do not have the colorful gorget; their underparts are pale gray to pale buff with darker flanks. Their tail is shorter than the male's and only slightly forked. The tail's upper surface is olive green to bronzy green and all but the central pair of feathers have a wide black bar near the end and white tips.

Subspecies R. v. koepckeae of the oasis hummingbird is known only from its type locality in northwestern Peru's Department of Piura. The nominate R. v. vesper is found from just south of there through Peru into Chile's Tarapacá Region. R. v. atacamensis is found further south in Chile between the Atacama and Santiago Metropolitan regions. Their range expansion south of Atacama has occurred since the 1960s. The species inhabits a variety of coastal and near-coastal landscapes including arid scrublands and their oases, riparian zones, agricultural areas, and gardens. In elevation it ranges from near sea level to 3,800 m (12,500 ft) but in most of Peru it is mostly found below 2,600 m (8,500 ft).

The oasis hummingbird forages for nectar at all heights of vegetation and also eats small arthropods, but details of its diet are lacking.