Rats are tunnelling under the guinea pigs' cage. I have blocked up the holes with stones but every morning there are new holes. They live outside in the garden. Can I put poison down the holes and then block up the holes so that the gps' can't go down? Or might the rats bring the poison back up and endanger the pigs? (I hate having to poison the rats, but I can't think what else to do. Can't have large quanities of rats about - last year they ate a whole nest full of baby birds and my daughter plays in the cage with the pigs.)