Bird of Prey

I've no idea what bird this is. I'm not familar with Colorado's wildlife. This guy (gal?) was floating on a thermal, hanging around a particular rock formation at the Red Rock Park along with its mate.

This is not an awesome picture - my big "lens" don't have a huge range, so to give ya'all a bigger, more visible image (click on it), I cropped and lightened it - which eroded the picture quality. One of the downsides of using a digital camera.

If anyone wants to take a stab at figuring out what this bird is, please share.