Thank you @papa-pepper for sharing your observation of tail shaking behavior as a defense/warning mechanism. What is fascinating is that even squirrels do it.
An interesting AAAS study of Pacific rattlesnakes and California ground squirrels by Matthew Barbour and Rulon Clark has found that when squirrels are confronted by rattlesnakes, the ones that vigorously shake their tails are less likely to get bitten. The effect extends to other squirrels nearby who notice and imitate the first squirrel's signals: they are also safer from attack. [http://www.sciencemag.org/news/2012/07/tail-shakes-scare-snakes link includes video, but it may be behind a pay wall: I couldn't see it]