A robin perching on the sunflower feeder

For many years I was under the impression that robins are unable to feed from certain bird feeders, where they have to land on metal perches. I thoughtr it was something to do with the shape of their claws. 

A couple of years ago when we first bought this type of sunflower feeder, an anti-squirrel design, where a too heavy weight of an animal on the perches slides the aperture down over the four feeding holes. Since then I've observed this feeder working very well and no squirrels have ever successfully fed from this type.

Then I noticed that robins tried to fly close to the perches and grab food from the holes. Eventually they tried landing very briefly, and began to stay  fractionally longer after getting this practice. Now they can land and stand and look around as well as grab seeds to take away. How clever they are.