Adult males of the nominate subspecies have metallic bronze green upperparts that are duller and darker on the crown.

Their chin, throat, and chest are bright metallic yellowish emerald green and the vent area and undertail coverts are deep cinnamon rufous.

[5] The nominate subspecies of buff-bellied hummingbird is found year-round from northern Belize and northwestern Guatemala north to Tabasco, Campeche, and Yucatán in southeastern Mexico.

[5][6] The buff-bellied hummingbird feeds on nectar from a very wide variety of flowering plants, shrubs, and trees including those with non-tubular blossoms.

It is very territorial, vigorously defending feeding sites including sugar water feeders from other hummingbirds and some insects.

[5] The buff-bellied hummingbird's breeding season in south Texas is mostly between late March and early August but nesting has occurred both earlier and later.

The nest is a cup of plant and other fibers such as thistledown, grass, hair, and nylon bound with spiderweb and covered with lichens and bark shreds.

When chasing other hummingbirds it gives a "long, low call with notes repeated rapidly (see-see-see-see-su-su)"; this vocalization has also been described as the species' song.