The Lesser, or Western Goldfinch has two subspecies, black-backed and green-backed, which have an overlapping range in the Rocky Mountains and even into California, where they may run into the Lawrence's Goldfinch, their uncommon cousin.
These little birds always seem to be peppy. They share at the feeders, which means they're often pushed out of the way by house finches and sparrows, but their love of thistle seeds is second to none (except probably to the American Goldfinch).
They have a large population at the Desert Botanical Gardens in AZ, where their thistle diet has been effectively substituted for the wild sunflowers. They didn't seem to mind at all.
My Arizona family had never seen one at the house, despite maintaining several bird feeders year-round. Once they displayed thistle seed, they had over a half-dozen goldfinches flitting about within 48 hours.