Why the (un)caged bird sings?

I have two feeders like this one. They are designed so the birds can hang on the outside and pull the seeds through the mesh – and because they are spheres they can get all of it right to the end. However, somewhere along the way the lid for this one was lost. I keep filling it, and the birds eventually decided it’s easier to drop inside for their meal, as demonstrated here by this white crowned sparrow. It’s telling that the one with no lid empties at twice the rate (or faster) than the one they have to use in its intended fashion.