I have two male rats, Angus (hooded rat) and Pudge (albino), that live happily together in a big two-story rabbit cage.
Angus is very affectionate when I take him out to hold him, but he won't stop nipping my hand every time I put it into the cage -- for example, to remove the food bowl, scoop out dirty litter, adjust the exercise wheel, etc. Not hard enough to break the skin, but still...
I'm assuming it's a territorial thing, but he goes out of his way to race over and get me. Every time. If it matters, I don't hand-feed treats, either -- they go into the food bowl.
Probably a long shot, but I'm hoping someone might have a suggestion.
"Dogs give and give and give. Cats are the gift that keeps on grifting." –Bradley Trevor Greive
What about giving him something delicious to gnaw on while you are doing cage maintenance? One of my males was very naughty through the cage bars but a doll in your hand. Everyone would come over, reach through the bars to pet him and before I could get out, "Careful! He bites!" they would be recoiling with a bloodied finger. Every. Single. Time. I would try to think of a solution either from the positive side (giving him a positive alternative) or from the avoidance side (avoiding actions that provoke the nip). Good luck~ Let us know if you come up with a successful alternative!
Mix cayenne pepper with hand sanitizer and apply to your hand just before putting your hand in the cage. Let him nip you. Do this every time your(or anyone's) hand goes into cage for a week or so. This will teach him people who smell like hand sanitizer TASTE BAD! Then when he quits nipping just use the hand sanitizer without the pepper. CAUTION do not touch your eyes,or his!
If you don't want to use pepper, try any other 'flavor' you know he does not like.