The Black Jacobins primarily feed on nectar taken from a variety of brightly colored, scented small flowers of trees, herbs, shrubs and epiphytes. Many native and cultivated plants on whose flowers these birds feed heavily rely on them for pollination. The mostly tubular-shaped flowers actually exclude most bees and butterflies from feeding on them and, subsequently, from pollinating the plants. They also take some small spiders and insects - important sources of protein particularly needed during the breeding season to ensure the proper development of their young. Insects are often caught in flight (hawking); snatched off leaves or branches, or are taken from spider webs. A nesting female can capture up to 2,000 insects a day.

Hummingbirds are solitary in all aspects of life other than breeding; and the male's only involvement in the reproductive process is the actual mating with the female. They neither live nor migrate in flocks; and there is no pair bond for this species. Males court females by flying in a u-shaped pattern in front of them. He will separate from the female immediately after copulation. One male may mate with several females. In all likelihood, the female will also mate with several males. The males do not participate in choosing the nest location, building the nest or raising the chicks.

The female Black Jacobin is responsible for building the cup-shaped nest out of plant fibers woven together and green moss on the outside for camouflage in a protected location in a shrub, bush or tree. She lines the nest with soft plant fibers, animal hair and feather down, and strengthens the structure with spider webbing and other sticky material, giving it an elastic quality to allow it to stretch to double its size as the chicks grow and need more room. The average clutch consists of two white eggs, which she incubates alone.

The female alone protects and feeds the chicks with regurgitated food (mostly partially-digested insects since nectar is an insufficient source of protein for the growing chicks). The female pushes the food down the chicks' throats with her long bill directly into their stomachs. As is the case with other hummingbird species, the chicks are brooded only the first week or two, and left alone even on cooler nights after about 12 days - probably due to the small nest size. The chicks leave the nest when they are about 20 days old.

The song of the black jacobin is a rapid, oscillating tune that resembles a cricket’s more than a bird’s, and can be heard by humans. But the song reaches a pitch higher than the known hearing range of any other species of bird, and it’s unclear whether black jacobins themselves can hear each other’s songs. Perhaps the birds have exceptional hearing, or maybe their vocalizations aren’t meant to be heard at all, say Claudio Mello of the Oregon Health and Science University in Portland and his colleagues, who recorded the birds’ song.

The black jacobin shares its forested habitat with up to 40 other hummingbird species and subspecies. The researchers speculate that the jacobins evolved high-frequency communication and hearing to identify each other amid the avian hubbub.