I've not done much wildlife stuff (aside from the peahen shot I did for the print exchange, but that thing was so tame it was all I could do to keep it out of my sandwiches).
However I was thinking about it recently, and found M42 lenses in rediculous focal lengths cost next to nothing. A trip to a local 2nd hand dealer returned a decent 400mm prime lens for 25 pounds. Adding a teleconverter ups that to 800mm for an all in cost of <30 pounds (shop around and you'll get a camera thrown in for free if you don't already have an M42 compatable!).
This stuff is so cheap, it's great fun at pocket money prices.