The perch spins. The stem is slide adjustable along the shell to align the window cutout with the seed ports. The counterweight is set for small birds and when squirrels or larger birds land, the shell spins. No squirrel has ever balanced on it because it spins like a bicycle wheel on an axle. After the squirrel is gone the weight knob rocks the shell back into feeding position and aligns the window with the seed ports.
Many feeders incorporate counterbalance levers or other means to close access to seed ports.
Where they tend to fall short is the closure is isolated to a single location and only works when weight is applied directly on the mechanism. The animals quickly figure out they don't need to touch the lever at all, only just reach down from above, over from the side, or by other ingenious methods.