The male has a green pileum and bright red throat, iridescent gorget with elongated lateral plumes, bluish upper parts, and the rest of the underparts mostly greyish white. Compared to other small hummingbirds, which often have a slender appearance, the bee hummingbird looks rounded and plump.

Female bee hummingbirds are bluish green with a pale gray underside. The tips of their tail feathers have white spots. During the mating season, males have a reddish to pink head, chin, and throat. The female lays only two eggs at a time, each about the size of a coffee bean.

The bee hummingbird has also been described as "the smallest dinosaur".[7] This characterization is based upon the recognition that birds are, in fact, a living form of dinosaurs (or, strictly speaking, avian dinosaurs), and no smaller bird or non-avian dinosaur has been found in the fossil record.

Starting in 2010, Kösen received Gamma Knife treatment for his pituitary tumour at the University of Virginia Medical School and was provided with medication to control his excessive levels of growth hormone. It was confirmed in March 2012 that the treatment was effective in halting Kösen's growth.