Appearance

Amazilia are a disparate group of species, but generally they are medium sized hummingbirds with more or less straight bills that are red basally, at least on the mandible. Blue-tailed Hummingbird is a small Amazilia. The bill is black above, but the mandible is red, with a dark tip. The body plumage is primarily green, with a purplish rump and uppertail coverts, and a deep blue tail. Also, the bases of the outer secondaries and inner primaries are rufous. Females are similar to males, but duller, with a grayish belly, and with reduced rufous on the wings.

Blue-tailed Hummingbird is similar to Berylline Hummingbird (Amazilia beryllina), but the tail of Berylline is deep rufous or bronzy, not deep blue. Blue-tailed Hummingbird also is very similar to Steely-vented Hummingbird (Amazilia saucerrottei), but Steely-vented Hummingbird lacks the rufous on the inner remiges. There also is little geographic overlap between Blue-tailed and Steely-vented hummingbirds.

The following description is based on Ridgway (1911), and refers to nominate cyanura; see also Geographic Variation:

Adult male: Crown, hindneck, back, scapulars, and lesser wing coverts deep metallic green. Rump dull purplish bronze, becoming dark metallic bluish on uppertail coverts. Rectrices dark metallic violet blue. Remiges dusky; secondaries and inner primaries chestnut or rufous chestnut, broadly tipped with violet dusky. Underparts mostly bright metallic green, sometimes with very narrow and indistinct dull whitish margins to the feathers. Undertail coverts dull steel blue.

Adult female: Similar to adult male, but duller. Feathers of underparts, especially on chin, throat, and breast, usually narrowly margined with whitish. Belly mixed with dull buffy whitish; undertail coverts more grayish. Rump duller, less purplish.

Undescribed.

Iris: dark brown

Bill: black; base of mandible reddish

Toes: dusky

Bare parts color data from Ridgway (1911) and Howell and Webb (1995).

Total length: 9 cm (Stiles and Skutch 1989), 9.5-10 cm (Howell and Webb 1995)

Linear measurements (from Ridgway 1911):

cyanura

male (n = 4)

wing length: mean 52.9 mm (range 51-54 mm)

tail length: mean 28.9 mm (range 27.5-30 mm)

culmen length: mean 18.8 mm (range 18-19.5 mm)

female (n = 3)

wing length: mean 51 mm (range 51-51 mm)

tail length: mean 28.7 mm (range 28-30 mm)

culmen length: mean 18.8 mm (range 18.5-19 mm)

guatamalae

male (n = 3)

wing length: mean 54.5 mm (range 53-56 mm)

tail length: mean 30.8 mm (range 30-31.5 mm)

culmen length: mean 18.8 mm (range 18.5-19 mm)

female (n = 2)

wing length: mean 51 mm (range 50-52 mm)

tail length: mean 30 mm (range 30-30 mm)

culmen length: mean 19.7 mm (range 19.5-20 mm)

Mass: male, 3.9 g (n = 1; Dunning 2008); 4.5 g (n = ?; Stiles and Skutch 1989)