Little brown bird with a long, wispy tail found at high elevations near treeline in the Andes of southern Ecuador and northern Peru. White eye-ring, lack of an eyebrow, and brown (not chestnut) upperparts help separate it from White-chinned Thistletail; also note range. Mouse-colored is found alone or in pairs in dense shrubbery and stunted forest. Typical song is a rising, piping series that quickly accelerates into a trill.