Rivoli's Hummingbird is a large hummingbird with a long, straight bill. This species is sexually dimorphic. Males are very dark, with a small white spot behind the eye; the forecrown is iridescent purple, the upperparts are dark green, the gorget is glittering green, and the rest of the underparts are blackish. The tail of the male is bronze green, and is slightly notched. Female Rivoli's Hummingbird lacks the iridescent gorget and crown; the upperparts are green, and the underparts are pale grayish white. The outer rectrices have a broad blackish subterminal band and small white tips.
Male Rivoli's Hummingbird is distinctive within its geographic range, and is unlikely to confused with any other species. Males of allopatric Talamanca Hummingbird (Eugenes spectabilis) of Costa Rica and Panama are similar, but its gorget is a bluer shade of green, and the underparts are dark green (not black).
Female Rivoli's Hummingbird is similar to the female of Blue-throated Hummingbird (Lampornis clemenciae; Colibrí Serrano Gorjiazul), but Rivoli's Hummingbird has longer bill, has only a white postocular spot (Blue-throated has a longer white postocular streak), and smaller white tips to the outer. The bill of starthroats (Heliomaster) are even longer than the bill of Rivoli's Hummingbird; starthroats also have white flank patches, and longer white postocular streak.
The following description is based on Ridgway (1911):
Adult male: Forecrown blackish, glossed with green or bluish green. Crown metallic violet or purple. Nape, sides of crown, and auriculars velvety black to bronze green. Postocular spot white. Rest of upperparts, including rectrices, bronze green or golden green; rectrices sometimes pale grayish at tip. Remiges dark brownish slate or dusky, faintly glossed with purple or purplish bronze. Chin and throat (gorget) brilliant emerald green, the gorget extending farther back along the sides of the throat than in the center of the throat. Breast velvety black, shading into bronze green on lower breast and to grayish brown or sooty grayish on belly and flanks. Tibial tufts white. Undertail coverts light brownish gray.
Adult female: Upperparts, including the two central pairs of rectrices, metallic or golden green; crown duller (forecrown sometimes dull grayish brown). Postocular spot white. Auriculars dusky. Basal half of the three outer pairs of rectrices bronze green, with a black subterminal band and a brownish gray or grayish brown tip, broadest on the outer pair. Remiges dark brownish slate or dusky, faintly glossed with purple or purplish bronze. Underparts brownish gray or buffy grayish, with a bronze green wash on the sides. Feathers of chin and throat with paler margins or margins of dull grayish white. Tibial tufts white. Undertail coverts brownish gray (sometimes glossed with bronze green).
Immature male: Shows a mix of characters of the plumages of the adult male and adult female. Crown partially violet, throat only partially green, and breast slightly intermixed with black.
Immature female: Similar to adult female, but feathers of upperparts narrowly tipped with pale grayish buff; underparts slightly darker and more or less suffused with pale brownish buffy.
Rivoli's Hummingbird probably follows the Complex Basic Molt strategy, although molts in hummingbirds require further research (Howell 2010). The Preformative Molt is partial, involving the body plumage but not the flight feathers, and is very protracted (Powers 2013). The Definitive Prebasic Molt is complete; in the northern migratory populations, molt is initiated on the breeding grounds, but is completed on the wintering grounds (Powers 2013). In southern Mexico, molt has been recorded from April to July, with heavy molt on the head and body in April (unpublished data).
Iris: dark brown
Bill: dull black
Bare parts color data from Ridgway (1911).
Total length: 12-13.5 cm (Howell and Webb 1995)
Linear measurements (from Ridgway 1911):
male (n = 31)
wing length, mean 73 mm (range 69.5-76 mm)
tail length, mean 43.2 mm (range 40.5-48 mm)
culmen length, mean 27.4 mm (range 25.5-31 mm)
female (n = 14)
wing length, mean 68.7 mm (range 66.5-70.5 mm)
tail length, mean 39.4 mm (range 37.5-41.5 mm)
culmen length, mean 29.1 mm (range 27-30.5 mm)
Mass: male (n = 47), mean 7.90 ± 0.49 g (range 6.81-9.55 g; Powers 2013); female (n = 14), mean 7.34 ± 0.45 g (range 6.74-8.10 g; Powers 2013)