Black-legged Dacnis forages from the midstory to the canopy. Apparently is at least partially nomadic, with poorly understood altitudinal and perhaps latitudinal movements, presumably understand in search of favorable foods (Willis 2003).
This dacnis often is encountered in flocks of 2-5 in the morning, joining larger mixed-species flocks later in the day. It sometimes associates with the very similar Blue Dacnis (Dacnis cayana) (Collar et al. 1992, Ridgely and Tudor 2009).
There is little information on territoriality in Black-legged Dacnis. Nests tend to be found in a rather close proximity to each other, and in close proximity to the nests of other species of birds, with no aggressive behaviors observed among pairs or between species (Kirwan 2009, Whittaker et al. 2010).
Black-legged Dacnis is at least socially monogamous (Whittaker et al. 2010). Although they nest, at least at times, in loose groups of conspecific pairs, there have been no reports of extra-pair copulations, and no description of courtship displays. Males have been reported regularly following females from tree to tree (Zimmermann 1995), and courtship feeding away from the nest has also been observed (Kirwan 2009).
Social and interspecific behavior
Black-legged Dacnis has not been observed exhibiting aggressive behavior toward other bird species. It commonly is found in pairs (Parrini and Raposo 2008, Aximoff and Freitas 2009) or in small groups, at least some of which may be family groups (Whittaker et al. 2010). It is a regular of mixed-species flocks (e.g., Gonzaga 1983, Isler and Isler 1987, Mallet-Rodrigues et al. 2007, Brandt 2008). In the early morning it has been observed in small groups, joining large mixed-species flocks including other tanagers and becards by mid-morning (Gonzaga 1983). Specific flock associates include Blue Dacnis (Dacnis cayana) and Fawn-breasted Tanager (Pipraeidea melanonota), as well as Green-backed Becard (Pachyramphus viridis), Crested Becard (Pachyramphus validus), Red-eyed Vireo (Vireo olivaceus), Chestnut-vented Conebill (Conirostrum speciosum), Yellow-backed Tanager (Hemithraupis flavicollis), Flame-crested Tanager (Tachyphonus cristatus), Palm Tanager (Thraupis palmarum), Green-headed Tanager (Tangara seledon), Red-necked Tanager (Tangara cyanocephala), and Buff-throated Saltator (Saltator maximus) (Gonzaga 1983, Collar et al. 1992, Mallet-Rodrigues et al. 2007, Parrini and Raposo 2008,).
No predation events on Black-legged Dacnis have been reported.