Scarlet-thighed Dacnis is very active and warbler-like (Restall et al. 2006, Ridgely and Tudor 2009). Both adults feed the young, and at least the female will aggressively chase other passerines away from active nests and recently fledged young (Skutch 1962).
There is no information available on the spacing of breeding pairs in this species. A female with nestlings and a recent fledgling defended the nest tree against other small to medium sized passerines, repeatedly flying angrily at a Yellow-bellied Elaenia (Elaenia flavogaster), and displacing a Clay-colored Thrush (Turdus grayi) while giving a low, nasal cry (Skutch 1962).
There is basically no information on sexual behavior in this species. Scarlet-thighed Dacnis presumably is at least socially monogamous. Both adults feed the young, but possibly females provision the young more often (Skutch 1962, Wetmore et al. 1984).
Social and interspecific behavior
Scarlet-thighed Dacnis typically is seen in mixed-species foraging flocks, but also can be solitary, as pairs, or in small conspecific groups (Haffer 1975, Ridgely and Tudor 1989, Parker and Carr 1992, Robinson 1999, Restall et al. 2006). Skutch (1962) reported it as usually traveling in loose flocks, but also often solitary, and Slud (1964) reported it as regularly occurring in small groups from several to about 15 individuals. It often is seen with Neotropical migrant warblers and other species of resident tanagers (Greenberg 1981). It often associates with Sulphur-rumped Tanager (Heterospingus rubrifrons) in mixed-species canopy flocks, and often joins Blue Dacnis (Dacnis cayana) at fruiting trees (Skutch 1962, Robinson 1999). It has been observed aggregating with twelve other frugivorous passerines at fruiting Cecropia and Ficus trees (Daily and Ehrlich 1994), including Clay-colored Robin (Turdus grayi), Tennessee Warbler (Oreothlypis peregrina), Green Honeycreeper (Chlorophanes spiza), Scarlet-rumped Tanager (Ramphocelus passerinii costaricensis), Speckled Tanager (Ixothraupis guttata), Bay-headed Tanager (Tangara gyrola), Silver-throated Tanager (Tangara icterocephala), Golden-hooded Tanager (Tangara larvata), Blue-gray Tanager (Thraupis episcopus), Palm Tanager (Thraupis palmarum), and Thick-billed Euphonia (Euphonia laniirostris). This study investigated the effects of intraspecific interactions on the foraging behavior of other frugivores, and found that that Sclarlet-thighed Dacnis was relatively unmolested compared to other species observed (Daily and Ehrlich 1994). During these observations, Scarlet-thighed Dacnis was subordinate to Clay-colored Robin, Blue-gray Tanager, and Scarlet-rumped Tanager on single occasions, and to Silver-throated Tanager twice, and showed dominance towards Silver-throated Tanager and Bay-headed Tanager on single occasions (Daily and Ehrlich 1994). A female with a nestling and a recent fledgling repeatedly flew angrily at a neighboring Yellow-bellied Elaenia (Elaenia flavogaster), and darted toward a Clay-colored Thrush with a low, nasal cry putting the thrush to flight (Skutch 1962).
Although no direct predation events have been reported, an excited conspecific flock continually vocalized near a small forest falcon (Micrastur sp.) plucking and eating a small bird (Wetmore et al. 1984).