re: Discovered a neighbor has a corn feeder 20 feet off my property line.

If you catch him shooting across the line, it's an issue.

If not, set up a blind on your side to take advantage.

You could also set up your own feeder near his to double down on the attractant.
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Honestly, there are lots of deer. Being good neighbors (say a kid mans the stand and shoots across) is most important.

Otherwise, you may find a lot more wardens called