Medium-sized hummingbird, male mostly green with bold white throat, white line behind eye, and blue crown. Gray tail in the western parts of range, blue in the east. Female not quite as colorful but still striking: bright buffy underparts with black cheek and white line behind eye. She is very similar to female Purple-throated Mountain-gem but has duller bronzy tail; only safely identified with a good view. A forest hummingbird that also comes out to edges and gardens, visits feeders.
The grey-tailed mountaingem is about 12 cm (4.7 in) long and weighs about 5 to 6 g (0.18 to 0.21 oz). It has a medium-length straight black bill, dark cheeks, and a white stripe behind the eye. Males have mostly dark bronzy green upperparts with an emerald green crown and a black to bluish black tail. Their chin and throat are white, the sides of the neck and upper breast bright green, and the lower breast and vent area dark gray. Females have entirely bright green upperparts.
The white-throated mountaingem feeds on nectar from a variety of flowering plants. Males typically feed in the forest interior while females more often feed in shrubby areas. Males are territorial, defending flower patches.
