Like all trogons, has distinctive chunky shape and upright posture; large, round head, dark eye with colorful eye ring, stout bill, and broad rectangular tail. Both males and females are bright yellow below and dark above with a narrow diffuse band of white feathers separating the yellow underparts from the dark chest. Distinguished by dark eye with sky blue eye ring and solid black (not barred) tail feathers; outer rectrices graduated and broadly tipped with white. Adult males are solid black on head and chest with a variable amount of bluish gloss on nape and hindneck; back and upper wing coverts also glossed bluish to golden green. Wings are black with white edgings on the basal portion of the primaries. Adult females are similar, but duller yellow below and dark slate blackish-gray above, lacking the iridescent blue and green gloss of the male (see Detailed Description).
Easily distinguished from all other yellow-bellied trogons by 1) solid black tail feathers broadly tipped with white (not barred); and 2) dark iris with pale blue eyering. Of the seven other yellow-bellied trogons that occur in Central and South America, only the Black-throated Trogon (T. rufus) and Gartered Trogon (T. caligatus) are sympatric with T. melanocephalus. Both of these have barred outer rectrices. T. rufus males are green rather than bluish black above and females are brown rather than gray above, and the bill is yellowish rather than gray or horn-colored. T. caligulatus males have a yellow eyering and both sexes have vermiculated black-and-white upper wing coverts instead of solid black.
T. melanocephalus is most similar to the closely related (but allopatric) Citreoline Trogon (T. citreolus) of the Pacific coast of Mexico. T. citreolus has a yellow iris and the blue eyering is much darker and less conspicuous than that of T. melanocephalus, giving it a very different facial expression. Second, the white tips of outer rectrices are broader on T. citreolus, such that the underside of the tail appears almost completely white. Johnsgard (2000) also notes that T. citreolus is paler overall on the head, abdomen, breast, and upper wing. A possible hybrid was reported by Schaldach et al. (1997), who collected a male in 1961 on the Atlantic slope of Mexico (Sarabia, Oaxaca) that showed characteristics of both T. melanocephalus and T. citreolus. Notably, the bird had a yellow iris and dark eye ring like citreolus, but otherwise was indistinguishable from 19 other phenotypically "melanocephalus" males collected at the same location.
From Ridgway (1911):
Adult male: Head, neck, and chest uniform black or slate-black, the crown and hindneck sometimes faintly glossed with metallic bluish; back, scapulars, anterior lesser wing coverts, and upper rump bright metallic bluish green to golden green, usually more bluish next to black of hindneck and sometimes intermixed with violet-blue; lower rump and upper tailcoverts rich metallic blue, violet-blue, or bluish violet; four middle rectrices metallic bronze-green to bluish green (rarely blue or violet-blue), abruptly tipped with black, the inner web of second and third rectrices (from middle) wholly black; three lateral pairs of rectrices black, broadly tipped with white (this about 15-30 mm wide); wings (except anterior portion of lesser covert area) slate black, the longer primaries edged basally with white; sides and flanks blackish slate or sooty slate, the latter more or less tinged or intermixed with orange-yellow; rest of underparts rich orange-yellow (cadmium or deep chrome) fading into yellowish white anteriorly, where forming a more or less well-defined band against blackish of chest, the feathers of tibia and tarsus sooty blackish.
Adult female: Similar to the adult male but metallic coloring of upper parts replaced by slate color or blackish slate.
Immature male: Similar to the adult male but middle rectrices mostly dull black (the metallic coloring mostly confined to basal two-thirds of outer web of middle pair and basal outer edge of next two), lateral rectrices narrower at tip and with two or three white spots or bars on subterminal portion of outer web, and outer webs of secondaries and greater wing-coverts spotted and edged with white.
Immature female: Similar to the adult female but middle rectrices without distinct black tip, lateral rectrices narrower at tip and with outer web more or less spotted or barred with white, and secondaries edged with white.
Most trogons are thought to undergo a complete molt of body and flight feathers shortly after the breeding season ends and a second partial molt of head and some upper body feathers several months later, prior to the subsequent breeding season (Johnsgard 2000). In El Salvador, Dickey and van Rossem (1938) reported that molt of flight feathers occurred between August and late October (i.e., presumably after the breeding season). However, in the most detailed description of molt, Foster (1975) collected adults of both sexes in breeding condition that showed extensive molt of body and flight feathers. Six males and six females were collected in Costa Rica from April to August. Males: (1) Collected April 25; was approaching breeding condition (testis 6 x 3 mm) and undergoing moderate replacement of body feathers over the entire body. (2 – 4) Three males collected between 7 and 19 July had testes of 5, 6, and 7 x 5 mm, had worn flight feathers, and were undergoing light to moderate body molt. (5) Collected 11 August; breeding condition (testis 10 x 7 mm), molting lightly in all areas, rectrices 2 and 3 growing. (6) Collected 16 August; breeding condition (testis 7 x 5 mm), moderate to heavy body molt, rectrices 1-3 growing, rectrix 6 dropped, secondaries 1, 8, and 10 growing. Females: (1) Collected 18 July, breeding condition (largest follicle 2 mm), ensheathed body feathers in all areas; rectrices 2, 3, 4, primary 5, and secondaries 1, 7, and 10 growing. (2 – 6) Five females collected between 1 and 17 August had largest follicles between minute and 3 mm, possibly indicating approach of second laying period; body molt; and extensive flight feather replacement (dropped, growing, and new primaries, secondaries, and rectrices). These data indicate that Black-headed Trogons experience periods of overlap of molt and breeding (Foster 1975).
Like all trogons (and unlike all other birds), toe arrangement is heterodactyl with digits 3 and 4 pointing anteriorly and digits 1 and 2 pointing posteriorly (Johnsgard 2000).
Bare orbital ring: sky blue to pale whitish blue (Ridgway 1911); delft blue (Dickey and van Rossem 1938).
Bill: Male: greenish white to pale blue; Female: maxilla black with tomia light blue basally, mandible light bluish-horn (Dickey and van Rossem 1938); grayish to greenish horn-color (Ridgway 1911).
Tarsi and toes: Male: slaty horn-color; Female: light bluish horn-color (Dickey and van Rossem 1938).
From Johnsgard (2000):
T. m. melanocephalus:
Wing: male 130-146.5 mm (mean = 137.9 mm, N = 29); female 133-141.5 mm (mean = 136.4 mm, N = 18).
Tail: male 136-165 mm (mean = 148.2 mm, N = 29); female 138-155 mm (mean = 147 mm, N = 18)
Mass: male 74 -91.1 g (mean = 83 g, N = 4); female 82.7-85 g (mean = 83.9 g, N = 2)
Unidentified sample from Guatemala: 66-95 g, mean = 80g (Smithe 1966)
T. m. illaetabilis averages slightly larger in all measurements (Ridgway 1911, Table 1). Mass: Male 84-94.3 g (mean = 89.4 g, N = 5). Female 78-88.8 g (mean = 84 g, N = 6).