Why do squirrels wag their tails up and down

it could be something they do to attract squirrels of the opposite sex


After I have fed them. I have noticed them wag when they have finished eating, possibly indicating they are full, and/or to alert other squirrels that there is food here.
They wag when they are nervous or anxious, Californian squirrels force blood into the tail to increase tail temperature when threatened by a rattle snake because rattle snakes can see infra red. This does not occur with other snakes.