Foraging flies

Fly, a photo by andiwolfe on Flickr.

In the subalpine zone of the Snowy Mountains of Wyoming there were numerous flies foraging on the flowers of Asteraceae. I'm not sure if they were after pollen or nectar or both, but they sure were attracted to these plants.

I'm not sure what species of fly this is. It's much smaller than a common housefly, but it doesn't have the long proboscis of a hoverfly.