Just a bit of back ground on me. I help an old friend at his shop 2 days a week. We sell 1500 mice-pinks mice a week, and what ever rats.
I always try to get people to try rat pups if the corn is of good size. What you experience is not all that uncommon. Snakes eat 2-3 times then stop on the rats.
No idea why. They almost always will reaccept mice though. Then might take rats later again. I always tell people to try to change food types in very late spring thru mid summer when a snake is more likely to eat anything.
Possible causes. Just my opinions.
1. Rats vary in smell. Cage condition, bedding, even food.
2. How hungry was the snake? If very hungry more likely to strike on movement.
3. They just want mice again.
How to help in transition if having trouble.
1. Try f/t or live. Some like one over the other.
2. Get your rat, and ask for a hand full of mouse bedding in the bag/box. Give it a little time to pick up the odor.
2b. Wash the rat and dry well, then place in mouse bedding.
3. Just wash the rat. Sometimes just the rat odor is enough to turn them off. A more neutral odor can actually be good.
4. If you really really want them on rats, and it is large enough for a good meal, feed a small mouse. As it is finishing the snake almost always has its head up in the air. Hold a f/t or fresh killed rat pup just in front of the snake (trying to keep your had low, out of site,,, or use forceps) and as it opens its mouth, just get the head of the rat pup in its mouth.
After a few times try a rat pup. They seem to get use to eating it that way. If it fails, do the feed trick again, but this time hold the rat pup long enough so the snake has a fairly good hold on the rat. Then wiggle ever so slightly. Not to scare it, but to make the snake constrict the rat pup. You can also use bedding for scenting a little and decrease it the more times you use it. Let him pass a meal time a little. An extra 3-4 days of hunger can be a good motivator.
Good luck
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Thanks for reading.
Big Tom
www.herpzilla.com