"The Gilbertson PVC cylindrical feeder has proven to be an effective almost squirrel-proof bird feeder used by a wide variety of song-birds.  The perches are made of spring-steel, and they flop down whenever a squirrel tries to feed at either of the two feeder ports.  Squirrels have so much momentum when they jump onto a feeder placed at least 5 feet out from the trunk of a tree, and on wire at least 3 feet long, that they can't hold on to the perches."  [Carrol L. Henderson, nature and wildlife author/photographer.]