Mountain Bluebirds (Sialia currucoides) are uncommon visitors to Tavasci Marsh [Arizona], as they generally hang out in higher elevations, but this flock of about 250 showed up in mid-January, 1999.
Western Bluebirds (Sialia mexicana) are Winter residents around Central Arizona. They can usually be heard singing their "flight song" as in this sample while they're flying in small flocks overhead.