I just ordered a squirrel call. I'll report back if it works.I grew up hunting squirrels in mid-state California, however they are of the ground-burrowing type, so not to much worry with them going up trees. The local wheat and corn farmers with cattle loved us coming out to the local farms and shooting as many as possible, because the holes are often hidden in low brush, and the cattle would step in them and break their ankles. And the hawks perpetually circling overhead would take the free meals within 10 minutes.
What worked wonders if they were spooked was a squirrel call. It was a small handheld combination of bellows and a whistle, and when you flicked it with a finger it would make a barking noise that sounded a lot like a barking squirrel. If they were skittish, a few barks and invariably they would stop, stand straight up, and try to see where it was coming from. A 5-second pause was all that was needed. Might be enough to give you time to squeeze off a shot. I'm guessing the tree variety would respond the same. If you tape it to your rifle you can even flick it while looking down the sights.
JR