Uneaten rats.

[Creepy Alien]

I actually like that idea!

The issue I have is that some of the rats get cooled too quickly, as the warm water gets cold. It's not difficult, just very time consuming. I have a few lucky ones that won't anything if it's not warm enough on first attempt to feed.

I recently started using a hair dryer for this exact reason (one purchased just for this... a whole $8 ). It actually works great. Even a couple of longer term hold outs grabbed the rat immediately. Just make sure the rat isn't too hot.

... and you don't have to heat it for very long at all.