I was waiting for a croaking sound for a good while, too, but gave up. I have wondered if they are producing a sound we can't hear.

I have given the cresties fresh pear from our pear tree, though I am too lazy to want to clean the food processor thingy, so I just cut it up finely and mushed it with the back of a spoon. I have also given them avocado with the same treatment, but I think that's all, because bananas don't last long enough for leftovers in my house and much of the other fresh fruit we eat is citrus or grapes.