Mountain Man, St Giles-in-the-Fields, London

If their sound veers towards sombre, live Mountain Man are anything but. "This is our new single, we're so excited," they exclaim and burst into peals of unrestrained laughter. When they have to cut short "Play It Right" because of an attack of the giggles, you have the feeling you have been let into a rehearsal during after-school hours.

That said, the music created by these three young women who met in college at Vermont is full of subtle complexity – and only performable with the utmost concentration and skill. Three-part harmonies are begun often without the help of an instrument – and bar the delicate sparse strumming or slow picking of an acoustic guitar, it is their remarkable, ethereal voices that carry the melodies.

Their voices and blend of traditional folk, Appalachian folk and Americana make them a pared-down female Fleet Foxes, with story-telling so entrenched in the natural world, with song titles such as "Honeybee", "Animal Tracks" and "Buffalo".

Although their songs are short, they each cast a spell on a seated crowd that is both silent and enraptured. Haunting voices are alienating in their other-worldliness at times, but a handful of songs have developed the kind of songwriting that leaves you wanting to replay – the gorgeous "Animal Tracks", and the lilting sweetness of "Sewee Sewee", which recalls the soaring melodies of the folk chanteuse Alela Diane. Arrangements play to the strengths of, and highlight, each singer's distinct voice, as different instruments in an orchestra. In "River Song", Alexandra Sauser-Monnig uses complex breathing to provides the rhythm.

As they point out tonight, this is only their second tour of the UK. With a reception like tonight's, and such a performance that brings the stillness of their debut album, Made the Harbor, to life, there will be plenty more to come.