The small and short-tailed Black-legged Dacnis is a lowland denizen of the forest canopies of eastern Brazil. Like all species of Dacnis, the sexes are dichromatic. The male is black and turquoise blue, whereas the female is olive yellow, tinged with blue on the upperparts. The male is remarkably similar to the male of Blue Dacnis (Dacnis cayana); see Similar Species. Females of the two are completely different, however.
The male Black-legged Dacnis easily is confused with the male of the much more common, Blue Dacnis (Dacnis cayana). Their ranges overlap, with the restricted range of Black-legged Dacnis entirely within the range of Blue Dacnis, and the two species regularly co-occur (Isler and Isler 1987, Sick 1993). Black-legged Dacnis is smaller, has a shorter tail, a stouter and wider bill, and dusky to blackish tarsi, whereas Blue Dacnis has pink to reddish tarsi (Meyer de Schauensee 1970, Sick 1993, Whittaker et al. 2010, Hilty 2011). Blue Dacnis also has blue edges to the primaries, a slightly larger black throat, and the black of the back is more extensive and not confined to the mantle (Cassin 1864, Ridgely and Tudor 1989, Hilty 2011). The relatively drab female Black-legged Dacnis does not resemble the bright green and blue faced female Blue Dacnis, and when an adult pair is seen together the female can further help distinguish these two species (Isler and Isler 1987, Ridgely and Tudor 1989, Sick 1993, van Perlo 2009). Indeed, female Black-legged Dacnis is unmistakable in its range, being bluish above with a sooty back and cinnamon brown below. The female Black-legged Dacnis looks more similar to the female Scarlet-thighed Dacnis (Dacnis venusta), with the most prominent difference being that female Scarlet-thighed Dacnis has a buffy white throat, whereas the throat of Black-legged is more cinnamon brown. Also note that these species are completely allopatric, as Scarlet-thighed Dacnis occurs from Costa Rica south only to western Ecuador.
The sexes are highly dichromatic.
Adult male: Head, nape, and neck mostly greenish blue to turquoise; lores, area surrounding the eye, and short, narrow streak behind eye all black, forming a small mask. Narrow black throat patch. The mantle is black, framed with turquoise scapulars, forming a distinct triangle. The back and uppertail coverts are light turquoise to greenish blue. The tail is black, the outer rectrices edged blue. Wing coverts and inner remiges black, edged with blue; outer remiges black. Underparts are light turquoise from the breast to the undertail coverts (Hilty 2011).
Adult female: Dorsally brownish olive, with a greenish to turquoise blue wash to the forecrown, cheeks, scapulars, and rump (Hilty 2011). Ventrally dull buff colored. Though the sexes are obviously dichromatic to humans, they are likely more dichromatic than we can percieve, for both sexes highly reflect UV light and this color is also dichromatic (Burns and Shultz 2012).
Subadult: Very similar to the adult female, but with irregular patches of greenish blue or turquoise on the underparts.
Plumage descriptions based on Hilty (2011; adult) and Whittaker et al. (2010; subadult).
Tanagers that have been studied have either a Complex Basic Strategy or Complex Alternative Strategy (Ryder and Wolfe 2009). However, most tanagers only molt once a year, and this prebasic molt likely occurs after the breeding season (Isler and Isler 1987, Ryder and Wolfe 2009). Many species have been found to breed in subadult plumage (Isler and Isler 1987). The performative molt is partial in Dacnis, with body plumage and lesser, median and greater coverts replaced while rectrices, remiges, and primary coverts are retained (Ryder and Wolfe 2009). A subadult plumage (Formative?) is described for Black-legged Dacnis, and this plumage is retained during the first year (Whittaker et al. 2010). More specific information on molt and its timing is not available for this species.
Iris: dark reddish brown, dark brown
Tarsi and toes: black or dusky, perhaps duskier in females
Bare parts color data from Sclater (1863), Ridgely and Tudor (1989), Whittaker et al. (2010), and (Hilty 2011).
Total length: 11 cm (Isler and Isler 1987).
Linear measurements (from Hellmayr 1935; n = ?):
wing length: male, 60-62 mm; female, 59-62 mm
tail length: 34-38 mm (sexes combined?)
bill length: 11-12 mm (sexes combined?)
Mass: ca 14 g (Gonzaga 1983); male, mean 13.5 g (range 12-15 g, n = 4); female mean 13.9 g (range 11-15.5 g, n = 6).