I can't offer any advice about getting rid of rats, but I do know for a 
fact that your concern about the birds potentially suffering if you stop 
feeding them for a while is unfounded.  (Not that your concern about 
birds is unfounded, but that they'll be fine).  Birds are well equiped 
to forage for food for themselves, and so even if a reliable source of 
nutrition goes missing, they can and will just go elsewhere as need be.  
Even in your cutover area of Beltsville (near the Spickner's farm I 
gather), there are native sources like poison ivy berries and plenty of 
bugs for birds to eat instead.  Plus, migration is beginning for most 
species, so the birds that have been eating at your feeder all summer 
long are about to go on vacation to Central and South America anyway.  
So, no need to worry about the birds!  Here's hoping someone else here 
has rat ridding advice.

Laura Appelbaum,
Cloverly, MD