What are some types of squirrel-proof bird feeders?

Some types of squirrel-proof bird feeders are situated atop long steel poles or encased in mesh material that allows small birds in but keeps squirrels and large birds out. They are available on such websites as Birdfeeders.com and Duncraft.com.

A feeder mounted on a narrow steel pole is safe as long as the pole is kept clean and slick. Dirt on the pole allows organic growth that gives a squirrel the foothold it needs to climb. Other types of feeders employ a trap door strategy where the perch doubles as the trigger. The perch is unaffected by the weight of a small bird, but if a squirrel or large bird perches on it, the trap door closes on the feed, sealing off the food supply until the animal leaves.

If a feeder is meant for very small birds, Duncraft sells one that is encased in a wire cage large enough only for small birds to enter.