One thing that is never brought up is how Saruman first contacted Radagast. Radagast was described by Gandalf as not being much of a traveler unless driven by great need. Considering Radagast doesn't really play a part in battling Sauron, I can't see him just going to Saruman with a worry about Nazgûl or growing evil.
Surely Saruman himself must have come to Mirkwood to give Radagast his message for Gandalf. Some may say he could have used his crebain to send Radagast a message, but surely the crebain would have been a giveaway of Saruman's dodgy intention? Then again he does deem Radagast to be a fool.
So is there any textual evidence to say how Saruman first contacts Radagast?