Harris' Antelope Squirrels (Ammospermophilus harrisii) are very common here in Tucson, and they are most active in the springtime.
Harris' Antelope Squirrels adore bird seed and are regular visitors at (and under) my bird feeder. They will stuff their cheek pouches with bird seed and then race off to their holes to store it safely away. The holes closest to the bird feeder, about 10 feet away, have all been claimed by the larger, more aggressive Rock Squirrels (Spermophilus variegatus). Rock Squirrels are gray with big, bushy tails. The other common squirrel here in Tucson is the Round-Tailed Ground Squirrel (Spermophilus tereticaudus) which is a pale tan color and has a very thin tail.