Grey-tailed Mountaingem is represented here by hummingbirds surveyed at Cloudbridge Nature Reserve, Costa Rica. The study recorded this population under an older subspecies name and described different male and female plumage.
For the cinereicauda population, males have a blue forecrown, white throat and postocular stripe, and gray tail. Females have a white postocular stripe and cinnamon-rufous throat and underparts.
The survey recorded the cinereicauda population in secondary tropical montane cloud forest at Cloudbridge Nature Reserve.
An extinction-risk assessment has not been added yet.