I had neighbors who discovered nesting rats in their outside walls
during a renovation, they were advised by an exterminator that if they
want to continue feeding birds to use a "solid" seed feeder (Seed
cylinder is fused together by gelatin/etc.) for less fallout, and fewer
freeloaders. They had actually seen a rat dangling from their previous
(empty) sunflower feeder, looking in at their family room hungrily.
They claim that obvious rat visits are fewer (I see occasional dead ones
on my driveway, courtesy of a kitty Vigilante). There are however,
'possums and raccoons aplenty.

Hilary Barnes
Bellevue, WA