Like all hummingbirds, the Vervain is nectarivorous and insectivorous. It is presumably the legitimate pollinator of some flowers, but also ‘robs’ nectar from flowers such as Passiflora (Kay 1999), and uses its short bill to visit small, insect-pollinated plants. Females have also been seen probing and apparently ingesting mineral-rich dirt in March, presumably to obtain calcium and other minerals during reproduction (Graves 2007). Tiny flying insects are caught on the wing in the typical hummingbird fashion: either by sallying from a perch, or by hovering in a patch of flying insects, then catching several insects in succession in short, quick darting motions (CJC). There are no known observations of insects being gleaned from foliage.
Nectar is probably the main source of its calories, and it has been seen feeding from many different plants; see below. It generally feeds on smaller, lower-yield flowers than the larger hummingbirds it is sympatric with, although there is some overlap in the flower species utilized (Lack 1976). Defecation is, like in many hummingbirds, primarily of excess water obtained while feeding, but the feces also contain small hard pellets of insect matter (CJC).
A list of plants the Vervain Hummingbird is known to visit (arranged by reference): Cuphea ignea, Hohenbergia spp, Stenocereus hystix, Bauhinia divaricata, Calliandra pilosa, Acacia sp., Crotolaria falcata, Securidaca brownie, Croton linearis, Laguncularia racemosa, Miconia quadrangularis, Vaccinium meridionale, Asclepias curassavica, Cordia sp., Bourreria sp., Plectranthus blumei, Dunalia arborescens, Morinda royoc, Cephaelis elata, Lantana camara, Holmskioldia sanguinea (Lack 1976), Pachystachys lutea, Anacardium occidentale, Lamaireocereus hystrix, Opuntia spinosissima, Caesalpinia pulcherrima, Delonix regia, Ageratum sp., Hibiscus schizopetalus, Malvaviscus arboreus, Marcgravia brownei, Plumbago scandens, Hamelia patens, Ixora coccinea, Russelia equisetiformis, Nyctanthes arbor-tristis (Tyrrell & Tyrrell 1990), Aloe vera, Callistemon lanceolatus, Plectranthus blumei, Tamarindus indica, Stachytarpheta jamaicensis (C. Levy pers. comm.), Mimosa sp. (Wetmore & Swales 1931), Inga vera(Keith et al. 2003), Pentas sp. (Downer & Sutton 1990), Passaflora sp. (Kay 1999), and Coffea arabica (CJC).