Actually, kendallroad and hackableFM, the difference between squirrel and bird locomotion makes it practical, albeit not perfect, to target one or the other with an electric barrier by placement. Assuming the feeder to be pole-mounted (as opposed to suspended), locating the electric barrier on the pole, not the feeder platform, should achieve near perfect targeting. If the feeder is suspended, place the electric barrier on the arm supporting the feeder and/or use a smooth wire instead of a chain for suspension. You might even insulate the smooth suspension wire at both ends, energize it, and ground the suspension arm. Of course, you have to consider the excellent acrobatic jumping capabilities of squirrels.
I found this thread by searching for a solution for my own squirrel and bird problem, which is how to keep both of them out of my raised bed strawberry and vegetable crops. The squirrels cleared out the entire strawberry crop just as it was approaching perfection for shortcake. Screen can work, but make it too much of a bother to tend to and pick the crop.
I don't have a solution yet, but have been scheming about a very simple, low-power circuit to inject a string of low energy pulse into a grid of wires strung across the crop a few inches above the soil level. Not perfect coverage, by far, but perhaps enough to develop an aversion reaction in the crop robbers.
I'm still groping for info on appropriate energy/voltage levels that would be completely safe, but discourage the robbers. I am concerned that devices using auto ignition coils fed with 12 volts might be a bit too powerful for my intended targets. I also don't want a dead animal disposal problem in my urban setting and I don't want terrified grandchildren and angry parents. I regard this as a contest between me and the robbers that requires finesse, not a blood feud. Perhaps a motorcycle ignition coil or a flyback transformer fed with less than 12 volts and with an energy-limiting capacitor feeding the primary? Hans' German circuit looks like a good start. I'm a little worried about how to test the tolerability of the results by humans, how many tests would be required, and how to recruit testers, but I'll cross that bridge when I get to it.
By the way, kendallroad, if you google "electric squirrel fence" or the like you will find line, battery, or solar powered commercial units in the US$50 to $100 range for garden use targeted at small animals. I just hate to buy something I can make, but liability is a consideration. I assume that Hrishilkesh has already googled, "electric fence" and read the info on the websites of the many commercial unit manufacturers.
Be safe and have fun.
awright