Scarlet-thighed Dacnis is a small tanager with red irides and a sharp pointed bill, as is typical of Dacnis (Hilty 2011). The striking adult male is an unmistakable electric bright blue above and is black below, the only species in the genus that is black ventrally (Garrigues and Dean 2007, Hilty 2011). Also black is the forepart of the face, including the area around the eyes, giving the appearance of a blue half-hood. The thighs are red, but often are concealed (Stiles and Skutch 1989). The adult female is green above, with blue tints, strongest on the sides of the face, scapulars, and rump, and is buff below, with cinnamon thighs and crissum (Stiles and Skutch 1989).
The male Scarlet-thighed Dacnis is striking and is unlikely to be confused. with any other species. It is superficially similar to Blue-necked Tanager (Tangara cyanicollis), in that they both have a similar color pattern of a distinct blue hood and black bodies. However, the dacnis is smaller, the bill of the tanager is much thicker, and the dacnis has a black throat, black wing coverts, and blue scapulars whereas the tanager has a blue throat, silvery yellow-green wing coverts, and black scapulars (Ridgely and Tudor 1989, Hilty 2011). The female Scarlet-thighed Dacnis is more easily confused with other species, but can be identified by a bluish rump and unstreaked buffy underparts (Hilty and Brown 1986). The most likely species to be confused with the female Scarlet-thighed Dacnis are other females of other species of Dacnis, such as Blue Dacnis (Dacnis cayana), Black-faced Dacnis (D. lineata), and Viridian Dacnis (D. viguieri), as well as female conebills (Conirostrum) (Hilty and Brown 1986, Ridgely and Gwynne 1989, Ridgely and Greenfield 2001). The female Scarlet-thighed Dacnis is bluer above than the female of any other species of Dacnis species in its range (Hilty 2011).
The following description refers to the nominate subspecies, Dacnis venusta venusta. See also Geographic Variation.
Adult male: The anterior face is black, encompassing the eyes to a small post-ocular point; this area of black includes the forecrown, lores, and throat (Skutch 1962, Isler and Isler 1987, Hilty 2011). The crown, nape, auriculars, and neck are turquoise, surrounding the black of the front of the face, like a blue half-hood (Stiles and Skutch 1989, Hilty 2011). The back, including the scapulars, rump, and uppertail coverts are turquoise-blue (Skutch 1962, Meyer de Schauensee 1964); the longest uppertail coverts are black (Wetmore et al. 1984, Hilty 2011). Wings and tail black. The tibial feathering is multicolored, scarlet along the tops of the thighs, and black the rest of the way down the leg; but the scarlet of the thighs often is concealed (Isler and Isler 1987). All the underparts, except the bright scarlet thighs, are deep greenish black, including all of the breast, belly, and undertail coverts (Skutch 1962, Hilty 2011). The scarlet thighs are often hidden (Isler and Isler 1987).
Adult female: Typically greenish olive above, tinged with blue, the brightest blue on the sides of the face, scapulars, and rump, and dullest on the sides of the back (Wetmore et al. 1984, Stiles and Skutch 1989, Restall et al. 2006, Hilty 2011). The wings and tail are dusky (Hilty 2011). Females are grayish on the throat, to dull gray brown on the breast, to grayish olive on the lower breast, sides, and flanks. The breast is faintly tinged with blue. Belly and undertail coverts yellowish buff (Skutch 1962, Hilty 2011). The thighs have a tinge of red at the base (Skutch 1962).
Immature male: Much like the adult female, but duller and more gray on the head, with little or no blue tones (Skutch 1962, Wetmore et al. 1984). Advanced immature males have a more defined turquoise “half-hood”, scapulars, back and rump, but the mask is small and dusky and the breast sooty rather than the deep black with green tinges of the adult male. The center of the throat and underparts are dull gray-white, and undertail coverts tinged buff (Hilty 2011). In Costa Rica, immature males show adult plumage around September, but it is noticeably duller compared to adults during the first full breeding season (Hilty 2011).
Although adult male and female are obviously distinct, the are likely even more so to the birds themselves, for they are very highly dichromatic when considering avian visual abilities that extend into the ultraviolet spectrum (Burns and Shultz 2012).
There is very little information on molts of Scarlet-thighed Dacnis. 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 (Isler and Isler 1987), and this prebasic molt likely occurs after the breeding season (Isler and Isler 1987, Ryder and Wolfe 2009). Evidence from field studies and museum specimens suggest that Dacnis species have partial preformative molt, resulting in distinguishable molt limits (Ryder and Wolfe 2009). Many species have been found to breed in subadult plumage, and enlarged testis have been found in sub-adult plumage male Scarlet-thighed Dacnis (Wetmore et al. 1984, Isler and Isler 1987). Field studies and evidence from museum specimens indicate that Dacnis have a partial preformative molt (Ryder and Wolfe 2009). The extent of the preformative molt follows temperate patterns where the body plumage, lesser, median, and greater coverts are replaced with rectrices while the flight feathers and primary coverts are retained (Ryder and Wolfe 2009). In Costa Rica, immature males show adult plumage around September, but it is noticeably duller compared to adults during the first full breeding season (Hilty 2011).
The iris of Scarlet-thighed Dacnis is crimson to bright red to deep red in the male, and a duller dark red to reddish brown in the female (Hellmayr 1911, Skutch 1962, Ridgely and Tudor 1989, Hilty 2011). The bill is black, short, and pointed (Sclater 1863, Hellmayr 1911, Hilty and Brown 1986). The tarsi are dark gray or dusky. A male taken in Chiriquí, Panama had the iris bright red, bill and inside of the mouth black, the tarsus dull brown, and the toes and claws fuscous-brown; a female from this locality had a dark brown iris, the maxilla and the tip of the mandible dull gray with the gape yellowish white, the tarsus and toes neutral gray, and the claws dull brown (Wetmore et al. 1984).
The total length of the Scarlet-thighed Dacnis is 11.2-12.2 cm (Sclater 1863, Wetmore et al. 1984, Stiles and Skutch 1989, Restall et al. 2006). Mass measurements range from 15-17.1 g (Strauch 1977, Greenberg and Gradwohl 1980, Lindell and Smith 2003). A series from Hartman (1961) reported 6 female weights averaging 16.04 g (± 0.41 g) and 37 males averaging 16.12 (± 0.19 g), with 5 individuals from Hartman and Brownell (1961) averaging 16.02 (± 0.63 g). A breeding conditioned pair from Panama had a male 17.1 g and female 16.2 g (Strauch 1977). Bill length averages 8.8 mm (± 0.08 mm) across 20 individuals in Costa Rica (Daily and Ehrlich 1994). Other reports include a culmen length of 12.4 mm (Schoener 1965). The width of gape is 8.0 mm (Wheelwright et al. 1984). Wing length measurements include 66 mm and tail 43 mm (Sclater 1863). Flight muscles comprise 22.1% of the weight and leg muscles 6.3%, the wing load is 0.21, and the wing span/ wing chord ratio is 1.8 (Moermond and Denslow 1985). Erythrocytes measurements from Hartman and Lessler (1963) are 11.2 μ (±0.18) by 6.0 μ (±0.06), with the nucleus 4.8 μ (±0.13) by 2.1 μ (±0.04). A number of measurements on both subspecies were reported by Hellmayr (1911) and Wetmore (et al. 1984); the measurements for the nominate subspecies can be found in Table 2 below, with the measurements for Dacnis venusta fulginata found in Table 1 in the Geographic Variation section.
Table 1. Adult measurements of Dacnis v. venusta from Hellmayr 1911 and Wetmore et al. 1984
|n and Sex
||Panama, Costa Rica