Hi, this is our first year with birds and we were also having some issues with rats earlier this fall. Like you, we didn't have hardware cloth buried, in the ground, so they had pretty easy access to our birds. We lost three ducklings before finally getting it straightened out. This is what we did:The coop is 3 ft off ground next to side of shed as a windbreaker due to the high winds we have here in the winter (gale force winds) the shed breaks the wind for the coop. We didn't put the hardwire cloth on ground when building coop and pen which is our fault as that would have prevented rats from burrowing into pen area
1) went back around the perimeter of our pen and buried chicken wire a few inches deep. Will at least help keep them from getting in.
2) they were still around, so we set a bunch of snap traps. The first bait we used was peanut butter, but that didn't work too well as they often licked it off without setting the trap off. We finally settled on these cheap peanut butter flavoured taffy candies (picture below). Melt them in the microwave and then mash them onto the trigger. Works really well. in one night we caught 5 rats and one mouse in 6 traps.
Also, if you've got a wood floor on your barn, I'd say they're nesting under there. If your birds can't get in, it may be a good place to put bait. A friend of ours uses 2" pipe
connected in a T as a bird proof bait station that is easy to refill. Hope this helps
.