Cyanoloxia grosbeaks are medium sized cardinalids. As the English name suggests, all species have a large, very deep bill. Male Amazonian Grosbeak is primarily dark, blackish blue; the forecrown, an indistinct supercilium, and a small shoulder patch are brighter blue. The female is uniformly deep brown.
Amazonian Grosbeak closely resembles Blue-black Grosbeak (Cyanoloxia cyanoides). These two species are mostly allopatric, although perhaps they meet in eastern Colombia. As its English name suggests, Blue-black Grosbeak is darker, the plumage generally being dark blue with black accents. Both species are paler, purer blue on the forecrown, supercilium, and lesser wing coverts, but these areas are a brighter blue on Amazonian Grosbeak compared to Blue-black Grosbeak. Females of the two also are very similar, but female Blue-black is generally a brighter, more rufous than the the duller, brown female Amazonian.
Amazonian Grosbeak is also may overlap locally with Ultramarine Grosbeak (Cyanoloxia brissonii) in central Bolivia. These two species are segregated to some extent by habitat, as Ultramarine occupies more arid forest and scrub, rather than the humid forests where Amazonian occurs. The queet call note of Ultramarine is slightly different from the sharper, harsher tchit or chink call of Amazonian, and Ultramarine has a faster and higher pitched song. The bill of Ultramarine is shorter but even deeper than the bill of Amazonian, and has a more curved culmen. Ultramarine also is a little smaller than Amazonian. The bright blue patches in the plumage of the male Ultramarine are even brighter than the corresponding areas in male Amazonian, and Ultramarine has a light blue rump, whereas in Amazonian the rump is more or less concolor with the rest of the upperparts. Female Ultramarine is a paler brown than female Amazonian. There is no geographic overlap at all between Amazonian and Glaucous-blue Grosbeak (Cyanoloxia glaucocaerulea) of eastern South America; Glacucous-blue also is smaller and smaller-billed, and the male overall is much paler, brighter blue.
Amazonian Grosbeak is sexually dimorphic.
Adult male: Primarily a uniform dark blue. Lores and area around lower eyelid black. Forecrown and indistinct supercilium bright blue. Wing coverts, remiges, and rectrices black, edged with dark blue; lesser wing coverts bright blue. Underparts dark blue, becoming more blackish on belly and undertail coverts. Underwing coverts black.
Adult female: Upperparts dull brown. Remiges and rectrices blackish brown, edged with dull brown. Underparts paler brown.
Juvenile: Similar to adult female. Greater coverts probably darker brown; rectrices relatively pointed (Johnson and Wolfe 2018).
Amazonain Grosbeak follows the Complex Basic Strategy (Johnson and Wolfe 2018). In northern Amazonia, molting is from April to September, peaking in June (Johnson and Wolfe 2018). The first preformative molt is partial, involving the body plumage, the lesser and median wing coverts, at least some greater wing coverts, and tertials; but this molt does not affect that rectrices, remiges, or primary coverts.
Iris: brown or black.
Bill: black; in male, base of mandible of gray or silver.
Tarsi and toes: black.
Bare parts color data from Haverschmidt (1968), Novaes and Lima (1998), and Willard et al. (1991).
Total length: 14.5-15 cm (Herzog et al. 2016)
Linear measurements (from live birds; Johnson and Wolfe 2018)
wing length (chord): mean 80.1 mm ± 2.4 mm (range 72.0-85.0 mm; n = 39)
tail length: mean 66.7 mm ± 2.9 mm (range 62.0-71.0 mm; n = 34)
bill length (chord, nares to tip): mean 13.6 mm (n = 1)
tarsus length: mean 22.3 ± 0.9 mm (range 21.6-22.9 mm; n = 2)
wing length (chord): mean 75.3 mm ± 2.2 mm (range 71.0-79.0 mm; n = 48)
tail length: mean 63.1 mm ± 2.2 mm (range 59.0-68.0 mm; n = 46)
bill length (chord, nares to tip): mean 13.2 mm ± 0.4 mm (range 12.8-13.6 mm; n = 2)
tarsus length: mean 22.3 mm ± 0.5 mm (range 21.7-23.0; n = 5)
Mass: Brazil, male, mean 26.4 g ± 2.9 g (range 21.5-31.0 g, n = 52); female. mean 25.9 g ± 1.7 g, n = 64) (Johnson and Wolfe 2018)
Venezuela, male, mean 24.2 g ± 1.26 g (range 23.0-26.2 g, n = 6); female, mean 23.9 g ± 1.34 g (range 22.1-25.0 g, n = 4) (Willard et al. 1991)
Suriname, male, 24-29 g (n = ?), female 24 g (n = ?) (Haverschmidt 1968)
Peru, male, mean 27.9 g ± 2.11 g, n = 4); female, mean 24.4 g, n = 3) (Weske 1972); male, 26 g, 27 g, n = 2), (O'Neill 1974)