Hi,
I had rats in my garden a few weeks ago. I didnt want to poison them or particularly have to get them out of a trap (yeuch) but I heard they HATE the smell of peppermint so I bought 4 bottles of peppermint essence, diluted it and sprayed everywhere once a day for a few days spraying where I had seen them and obvious other places they had dug. I also stopped feeding the squirrel and birds at the same time as Mr Rat was getting food dropped from the feeders.
Ive not seen any rat in my garden since - although I did see a dead one in the street (I think they moved to a neighbours garden and probably got poisoned :red).
I know people with aviarys quite often have rat problems and they make sure the aviary is on concrete or slabs to stop rats burrowing in.