The grey-tailed mountaingem is treated as a species by the International Ornithological Committee (IOC) and BirdLife International's Handbook of the Birds of the World (HBW). However, the North American Classification Committee of the American Ornithological Society and the Clements taxonomy treat it as a subspecies of the white-throated mountaingem (Lampornis castaneoventris).Clements, J. F., T. S. Schulenberg, M. J. Iliff, S. M. Billerman, T. A. Fredericks, J. A. Gerbracht, D. Lepage, B. L. Sullivan, and C. L. Wood. 2021. The eBird/Clements checklist of Birds of the World: v2021. Downloaded from https://www.birds.cornell.edu/clementschecklist/download/ Retrieved 25 August 2021
