Oropendolas (Psarocolius) are very large, sexually dimorphic icterids with stout, sharply pointed bills. Oropendolas are predominately brown, black, or olive in color, and the outer rectrices are extensively yellow. Montezuma Oropendola has chestnut upperparts, with a black head and upper breast, and yellow outer rectrices. The bill is strikingly bicolored, black at the base with an orange red tip. There also is a large, pale blue area of bare skin in the sides of the face below the eye, bordered below by a smaller pink area. The sexes are similar in plumage, but the female is much smaller than the male.
Montezuma Oropendola overlaps geographically, from Mexico to Panama, with Chestnut-headed Oropendola (Psarocolius wagleri). Chestnut-headed Oropendola is smaller and paler brown than Montezuma, and has a pale yellow green bill, which is very different from the black and orange bill of Montezuma. Montezuma has chestnut upperparts with a black head, whereas Chestnut-headed has a pale chestnut head and black or blackish upperparts (Peterson and Chalif 1973). Chestnut-headed has a fully feathered head, lacking the bare blue and pink facial skin of Montezuma, but its iris is pale blue, not brown as in Montezuma.
There also is narrow geographical overlap, in Panama, with Crested Oropendola (Psarocolius decumanus). Crested Oropendola is very black, with a entirely pale bill, no bare facial skin, and pale blue irides.
The following description is based on Wetmore et al. (1984) and Jaramillo and Burke (1999); see also Ridgway (1902):
Adult male: Head, nape, upper back, throat, and breast black. Rest of upperparts (back, rump, and wings, including wing coverts) chestnut. Central rectrices blackish, and slightly shorter than outer rectrices; outer rectrices yellow. Basal half of outer web of primaries chestnut, the chestnut increasingly extensive on the bases of the inner primaries; primaries tipped blackish. Secondaries chestnut on outer web, blackish on inner web. Belly and flanks chestnut. Underwing coverts black.
Adult female: Similar to male, but smaller (see Measurements). The black of the head is duller and browner; the central rectrices also are duller and browner. The chestnut of the underparts sometimes is darker and blackish.
Immature male (First Basic): Similar to adult male, but smaller.
Juvenile: Similar to adult, but duller, with the black not glossy but instead dull, sooty black.
Little information, apart from the summary in Jaramillo and Burke (1999). The definitive pre-Basic molt is complete, and occurs after breeding. The extent of the first pre-Basic molt has not been documented. No pre-alternate molt is known.
Facial skin: bare facial skin below the eye, and extending from the mandible back to the rear edge of the eye, pale blue (but may be grayer around the eye); stripe of bare skin extending distally from the lower base of the mandible pink or pinkish
Bill: base of bill, including frontal shield, black; tip of bill reddish or orange
Tarsi and toes: black
Bare parts color data from Sturgis (1928), Land (1970), Wetmore et al. (1984), and Jaramillo and Burke (1999).
male, 39.5-47.5 cm (Wetmore et al. 1984), 50 cm (Stiles and Skutch 1989)
female, 34.7-39.8 cm (Wetmore et al. 1984), 38 cm (Stiles and Skutch 1989)
Linear measurements (from museum specimens, Wetmore et al. 1984):
male (n = 10)
wing length: mean 236.7 mm (range 218.8-261.0 mm)
tail length: mean 181.6 mm (range 170.9-200.2 mm)
bill length (culmen from base): mean 74.4 (range 71.5-81.1 mm, n = 9)
tarsus length: mean 54.5 mm (range 52.4-56.9 mm)
female (n = 10)
wing length: mean 197.0 mm (range 187.3-208.2 mm)
tail length: mean 153.6 mm (range 187.3-208.2 mm)
bill length (culmen from base): mean 58.8 (range 57.4-61.1 mm)
tarsus length: mean 45.2 mm (range 43.8-46.9 mm)
Linear measurements (from live birds, Webster 1997):
wing length (flattened): mean 267.6 ± 11.4 mm (n = 32)
bill length (culmen): mean 77.65 ± 3.28 mm (n = 33)
tarsus length: mean 67.37 ± 2.27 mm (n = 33)
wing length (flattened): mean 201.8 ± 9.7 mm (n = 50)
bill length (culmen): mean 60.01 ± 2.53 mm (n = 54)
tarsus length: mean 53.71 ± 1.54 mm (n = 56)
wing length (flattened): mean 233.2 ± 14.4 mm (n = 12)
bill length (culmen): mean 75.69 ± 3.27 mm (n = 14)
tarsus length: mean 66.03 ± 1.83 mm (n = 14)
Mass (from Webster 1997):
adult male, mean 515.9 ± 40.8 g (n = 33)
female, mean 245.8 ± 14.4 g (n = 58)
juvenile male, mean 418.8 ± 30.4 g (n = 14)