...an analogy might help answer this question: If I purchase a “waterproof” watch I expect the inner workings to remain dry when the outer case gets wet. Waterproof, however, doesn’t mean that water evaporates on contact with the watch – in other words, I expect the outer casing of the watch to get wet, but this isn’t really much of an issue to me because water, unlike a grey squirrel, in this situation, is quite harmless. A water-resistant watch on the other hand just seems to be able to deal with the occasional splash of water, it wouldn’t survive a good dunking!
Perhaps you see what I mean when I say that I don’t really know what squirrel proof means?
So what “might” a squirrel proof bird feeder do? A squirrel proof bird feeder would have to be a feeder that squirrels couldn’t gnaw and harm and that is highly unlikely to ever exist. Even the very best waterproof watches get wet, don’t they?
On the other hand, I know exactly what a squirrel resistant bird feeder is: this feeder should keep grey squirrels at bay and off the bird food. They should do no harm to the squirrel and they shouldn’t be over-engineered as that puts the birds off feeding and that, as they say, would be an own goal.
I don’t mind my squirrel resistant bird feeders getting battered and gnawed by grey squirrels as they can’t get to the bird food that’s inside them, and that’s all the resistance a good bird feeder needs to see off even the most persistent of squirrels.
An update: 22/6/2015
We may have found a squirrel-proof bird feeder after all...