That looks like an adult Harlan's RT to me. Look at this photo of a clear representative that looks very similar - http://www.azfo.org/gallery/2009/htm..._20090228.html. They even explain the unusual markings. There is a tiny bit of red in the tail. I thought that the photo above was a juvenile zone tailed hawk until I looked more closely at the barring on the tail, which looks more like a redtail. A juvenile zone tailed hawk has slightly thicker and a little less frequent barring. Notice also that the birds in both photos have barred undertail coverts, which is typical in Harlan's RTs, but not in zone tailed hawks (solid black).