A RWS rifle would be a good choice in either .177 or .22 cal. Be aware that once they're on to what you doing, you'll have to get up earlier each morning and wait for them to wake up. After first shot they tend to hide on top of tree limbs and hard to see and almost impossible to hit.
The Steiger model also has a suppressor and factory scope as well.
http://m.stoegerairguns.com/products/atac/index.php