Talamanca Hummingbird is nectarivorous, but its diet is not known in much detail; flowers that it is reported to visit include those of giant thistle Cirsium, Centropogon, various epiphytes (Cavendishia smithii, Macleania glabra; Ericaceae) and vines (Passiflora), bromeliads (Thecophyllum orosiense, Bomarea castaricensis), Fuchsia, Cestrum, sage (Salvia iodichroa), and flowers of other species of shrubs (Wolf et al. 1976, Stiles and Skutch 1989).
Talamanca Hummingbird presumably also feeds on small arthropods, , as do most if not all species of hummingbirds (Remsen et al. 1986).
The foraging behavior of Talamanca Hummingbird varies somewhat by season and sex. In breeding season, males defend territories around sites where giant thistle (Cirsium) is numerous, and defend at Centropogon at other times of the year (Wolf et al. 1976, Stiles and Skutch 1989). When foraging at Cirsium, the time budget of the male is perching 86.4%, foraging 7.3%, chasing 2.1%, and flight 0.8%, with males out-of-sight and unaccounted for during the remaining period (3.3%) (Wolf et al. 1976). The time budget for males feeding at Centropogon is similar: perching 85.1%, foraging 9.7%, chasing 1.6%, flight 0.2%, and out-of-sight 3.4%. Compared to other syntopic hummingbirds, male Talamanca Hummingbirds appear to spend more time perching; for example, Fiery-throated Hummingbird (Panterpe insignis) perches 43–70% of the time (Wolf et al. 1976). Males also forage in the canopy at flowering epiphytes and vines (Stiles and Skutch 1989). Females forage by trap-lining, frequently visiting clumps of Centropogon (Stiles and Skutch 1989).
Foraging for arthropods by Talamanca Hummingbird has not been described in detail; the closely related Rivoli's Hummingbird (Eugenes fulgens) primarily captures small arthropods in flight (Powers et al. 2010), with prey taken both from the air and from foliage (Powers 2013).