Starthroats (Heliomaster) are large hummingbirds with long, straight bills and tails that are square tipped or only slightly rounded. Most species are relatively dull in color, Plain-capped Starthroat being the drabbest member of the genus. The upperparts generally are dull green, with an irregularly shaped patch of white on the lower back. The auriculars are dusky, bordered above by a white postocular stripe and below by a white submoustachial stripe. The upper throat is gray; the gorget on the lower throat is glittering pinkish red to orangish red. Otherwise the underparts are pale gray, with sparse green mottling on the flanks, and a white tuft on the lower flanks. The belly is white; the undertail covers are dusky, the feathers with broad white margins. The tail feathers are green, with a blackish subterminal band and white tips. The sexes are similar. Immatures are duller, with little or no red on the throat.
Plain-capped Starthroat is similar to Long-billed Starthroat (Heliomaster longirostris), and these two species overlap geographically from southwestern Mexico south to northwestern Costa Rica. Long-billed Starthroat occupies more humid habitats, has a glittering blue or bluish green forecrown, has more rose colored gorget, and greener flanks.
The following description is based on Ridgway (1911), and refers to nominate constantii; see also Geographic Variation.
Adult: Sexes similar. Upperparts metallic bronze green, somewhat duller on the crown (especially the forecrown), and with a longitudinal patch or broad streak of white on the rump. Central pair of rectrices bronze green, dusky terminally; outer rectrices extensively blackish terminally, and with the inner web tipped with white. Remiges brownish slate or dusky. Postocular streak white. Auriculars and lores dusky. Broad white submoustachial streak. Chin sooty or blackish. Throat bright metallic red or purplish red (varying from orange red or scarlet to rose red). Underparts generally brownish gray, becoming white on the belly; undertail covers pale gray, with broad white tips. Flanks with a large, conspicuous tuft of silky white feathers.
Immature: Similar to adult, but throat dark sooty brown or dusky, the feathers terminally margined with grayish white.
Juvenile: Similar to immature, but feathers of upperparts narrowy tipped or margined with buffy,
Very little information. Wagner (1957) reported that in Mexico molt occurred primarily from July-September. Dickey and van Rossem (1938) examined two specimens that were "going through a complete molt" at the end of May in El Salvador, and also noted that the "postjuvenile molt" (preformative molt?) was "nearly complete" on two specimens from February and April.
Iris: dark brown
Bill: dull black
Bare parts color data from Ridgway (1911).
Total length: 11.3-13 cm (Ridgway 1911), 12 cm (Stiles and Skutch 1989), 12-13 cm (Howell and Webb 1995)
Linear measurements (from Ridgway 1911):
male (n = 15)
wing length: mean 66.9 mm (range 63-70.5 mm)
tail length: mean 34.1 mm (range 28.5-37 mm)
bill length: mean 34.5 mm (range 33.5-36.5 mm)
female (n = 10)
wing length: mean 65.7 mm (range 64-68.5 mm)
tail length: mean 32.6 mm (range 31-34 mm)
bill length: mean 35.5 mm (range 34-37.5 mm)
Mass: male, mean 7.4 g (range 6.5-9 g, n = 6; Johnsgard 1997); female, mean 7.2 g (range 6.8-7.2 g, n = 3; Johnsgard 1997)