Yep, this definitely looks like bird poo but it is actually a Two-tailed Swallowtail Butterfly caterpillar! These little critters have a voracious appetite and can decimate a citrus tree in no time. When this creature matures, it is a beauty and is the official Arizona State Butterfly!
I'm a transplanted New Yorker who loves the Tucson climate. On New Year's Day I have a bit of fun calling my (snowbound) east coast friends to announce that I'm drinking coffee while sitting on my patio. (: