The song of Scarlet-thighed Dacnis is described two to three scratchy, insect-like trills ending with a squeaky bzzeek bzzeek bzzeek (Stiles and Skutch 1989). The song lasts no more than several seconds in duration.
The calls of Scarlet-thighed Dacnis have been described as a metallic urp, zirp, rit, or wurt, a buzzy wuzt or rezt (Slud 1964), a chirping woit or wheit with an upward inflection (Stiles and Skutch 1989), and various scratchy inflecting bzik notes (Hilty 2011). Individuals excited by a nearby forest falcon (Micrastur sp.) plucking and eating a small bird continually vocalized wheu wheu with a curious metallic tone (Wetmore et al. 1984). A female defending her young uttered a low nasal cry (Skutch 1962).