Get rid of the mice pronto. If the bird table is one of those on a pole. you can put a bit of tin around the pole and prevent any access they may gain to the table. Cut up green peas tins etc. are ideal and free. Mice don't have very good toilet habits and it is the urine on the bird food that is dangerous. If you want to feed the mice, a bit of food under a half tin with a small hole cut will do them fine but be warned.... if you feed mice they will breed like... well... mice.
Someone has stolen your filter? Good grief! Is nothing sacred? You can actually make a cheap filter your self very easily.
One large tin - or even a plastic container with the top cut off, about 2 gallons. The iffy bit is making a hole low in the side so that a hose can be inserted without too much water spilling out. One water pump available from most garden centres pumps the water in the top of the tin. You will have filled the tin with filtration medium (glass wool etc.) It will be easy to clean the filter medium. You can hide hoses etc with flat stones/bricks and if you use a plastic container make sure the sun can't get to it or it will not last long.