Two days ago we bought "The Absolute Squirrel-proof Adjustable Bird Feeder" from Lee Valley tools in Ottawa. (Lee Valley does have a catalogue.) We have been pleasantly surprised to see that it works quite well.
Squirrels had been raiding our previous bird-feeder. The wooden top was made of two sections with a gap between them at the peak. This provided a handy place for the squirrels to hook their hind claws in as they hung over the edge of the feeder and feasted.
The new feeder has a very smooth top. There is nothing to grip. As the squirrels lean over to feed, they slide forward and topple off to the ground.
If they attempt to grab the perch from below, their weight causes the perch to lower, cutting off access to the seed. The photo with the squirrel behind the feeder shows the rear perch is lower than the front perch, and the seed inaccessible.
We can set the trigger to release the perch at three different weights. At the light weight it will accept Cardianals, Juncos and Sparrows. At the medium weight it accepts Black Birds, Mourning Doves and Blue Jays. It accepts heavier birds at the heavy setting. The sparrow at the feeder shows it works.