Amazilia are medium sized hummingbirds with more or less straight bills which are red basally, at least on the mandible. The bill of Cinnamon Hummingbird is extensively red, with only the tip black. This species has a simple plumage pattern: green upperparts and cinnamon underparts, with a tail that is mostly rufous.
No other species of hummingbird in the range of Cinnamon Hummingbird has uniform, cinnamon colored underparts.
The following description is based on Ridgway (1911), and refers to nominate rutila; see also Geographic Variation.
Adult: Sexes similar. Upperparts metallic bronze green. Rectrices deep cinnamon rufous or rufous chestnut; rectrices broadly tipped with dark metallic bronze, and the outer web of the outermost rectrix also is edged for most of its length with dark metallic bronze. Remiges dark brownish slate or dusky. Underparts cinnamon or dull cinnamon rufous, slightly paler on the chin and upper throat.
Juvenile: Similar to adult, but with rufous edgings to the feathers of the face, crown, and rump, and the underparts often are paler (Stiles and Skutch 1989).
Little information. In El Salvador, the "annual" (prebasic) molt is between February and early April (Dickey and van Rossem 1938).
Iris: dark brown
Bill: red with a black tip (male) or maxilla mostly black, with only the base and culmen red (female); maxilla of juvenile black
Tarsi and toes: dusky, dark brown
Bare parts color data from Dickey and van Rossem (1938) and Stiles and Skutch (1989).
Total length: 9.5 cm (Stiles and Skutch 1989), 10-11.5 cm (Howell and Webb 1995)
rutila and diluta, male (n = 33; Ridgway 1911)
wing length, mean 56.5 mm (range 52.5-60 mm)
tail length, mean 34.6 mm (range 31-37 mm)
culmen length, mean 21.2 mm (range 19.5-23.5 mm)
rutila and diluta, female (n = 17; Ridgway 1911)
wing length, mean 55.3 mm (51-58 mm)
tail length, mean 33.2 mm (range 31-37 mm)
culmen length, mean 21.6 mm (range 20-23.5 mm)
graysoni, male (n = 5; Ridgway 1911)
wing length, mean 68.1 mm (range 66-69.5 mm)
tail length, mean 43.5 mm (range 42-45.5 mm)
culmen length, mean 25.1 mm (range 23.5-27 mm)
graysoni, female (n = 4; Ridgway 1911)
wing length, mean 66.4 mm (range 66-67 mm)
tail length, mean 44 mm (range 42.5-45.5 mm)
culmen length, mean 25.7 mm (range 25-27 mm)
Mass: mean 5 ± 0.7 g (range 3.3-5.7 g, n = 14, sexes combined; Dunning 2008); mean 5.5 g (range 4.9-6.1 g, n = 8, sexes combined, graysoni; Grant 1965).