We have been waging a constant battle against 'Tree Rats' (grey squirrels) trying to get into our bird feeders.
We bought a 'Yankee Flipper' (by Droll Yankee) which successfully keeps the squirrels off, but I had not seen it flip a squirrel just bad timing, I guess.
Recently, I had let the batteries get discharged and they 'discovered' it... After recharging the batteries, I decided to let them feed for a few days, then turned it back on...