The first concern is that if they are getting in, so too can other predators who can do more damage than steal feed and/or spread disease. On that front, reassessing your security measures may be in order.
To address the feed loss - option one is to use a treadle style feed dispenser(s) - the birds are heavy enough to trigger the treadle and access the feed, smaller animals like mice are not. Alternatively, as most of the feed loss from mice, etc tends to come at night when the birds are not active but the rodents are, removing (or covering) feed from dusk to dawn can also help to limit the issue.
you can hang this from ceiling or pole or what-ever the heads eat with heads inside the bucket, so no mess and if hung from above the micce can't get in. I have one 5-gallon bucket form my 5- hens and it lasts about 3-4 weeks in the summer...winter i am sure this will change...but anyway it keeps food in bucket or in chicken so no wasted food or fat mice! = $$ saved - I used a 4" 90degree elbow & sawed off the lip of one end and inserted into a hole i cut in side of bucket so cut end of elbow is positioned about 3/4" off the bottom of the bucket. I made so it was very tight fit then added a bit of cauking around elbow and bucket to help hold in place and make it ito onepiece... then once all dried filled with feed and hung in coop - and we live in the forest with lots of rodents, but none eating my chickens food!
