The Bumblebee Hummingbird (Atthis heloisa) is an endemic bird to Mexico. Its natural history is poorly known, particularly its breeding biology. We provide novel and complementary information that enriches the biological knowledge on the species. We describe a nest of the Bumblebee Hummingbird found in Northern Oaxaca and characterize the nest site. The nest was mainly assembled with mosses and lichens; however, we recorded females transporting hairs of the invasive fern Pteridium sp. and seeds of the bromeliad Catopsis sp. for nest construction. Males were noisy and conspicuous while females were secretive; thus, we suggest considering differences in detectability among sexes when studying the species. Males exhibited courtship displays that consisted of several vertical movements over females while adopting a horizontal posture, erecting the tail, emitting a bee-like sound, and fanning out the feathers of the throat toward the females. The Bumblebee Hummingbird is considered a sedentary species, but we suspect it performs altitudinal movements in search of cloud forests during spring and summer and humid pine-oak forests during winter and autumn. Although the literature indicates that this hummingbird reaches breeding conditions between April and July, we recorded intense breeding activity only during January and February. It is not clear if such variation is regional or if the species could breed multiple times during the year.