Mountain Avocetbill is primarily nectarivorous, but there is little documentation of the plant species that it visits. Hilty and Brown (1986) describe the food plants as "tiny tubular flowers in thick shrubbery", and Schuchmann (1999) specifies "flowering Ericaceae, Onagraceae (especially Fuchsia), Rubiaceae, [and] Lobeliaceae". Also visits flowers of Centropogon (Campanulaceae) (Krabbe, in Ridgely and Greenfield 2001b).
Mountain Avocetbill also consumes small arthropods (Schuchmann 1999), as do most if not all species of hummingbirds (Remsen et al. 1983).