I had the same prob a few years ago the rats just ran out a quick snatch at the bate and back in to eat at there leasure giving me no time to shoot them with an air rifle,I thought about the shotgun but it was to near the shed and the chickens inside. I used a live trap cage and over three nights got them all I think. Diesel soaked paper stuffed down there hole works very well at keeping them out but they will just set up home somewhere else and still visit you chickens feed. Good luck they can be tricky b------S A ferret and a couple of terriers gives them a bit of a shock and is good sport.