There is a TED Talk on training crows to use a vending machine. The guy's ultimate idea was to train 6 wild crows to pick up scraps of garbage in a sports stadium, drop them into a special waste recepticle with an electronic eye, and the trashcan would reward the crow with a small food pellet of good quality.
Crows are known to teach other crows outside their immediate family skills that they have learned. In time, with no additional effort, those six crows could train the local population to clean a stadium after the people leave....for PEANUTS!
At the Black Hills Raptor Center, we have plans to eventually obtain a young crow. One thing I have heard from person after person that has trained them. Stay on your toes, do not stop teaching new behaviors, do not settle for the same behavior over and over. Or else the crow will get bored, skip to the end, demand payoff, and ruin everything!
At the Black Hills Parrot Welfare Center in Belle Fourche they have a hybrid African raven, African crow. His name is Murphy. One of his favorite games it to have you select a pebble from the driveway and toss it back into the driveway after showing it to him. He will sift gravel until he finds it and returns it to you. I was doing that with him one day when I was called away suddenly. When I came back a week later, Murph' flipped his food bowl over, pulled up my pebble and brought it to me cawing wildly! Greg says he wondered why Murph' had adopted a pet rock for the week. And get this: MURPHY DOES NOT PARTICULARLY LIKE ME. I have apologized for what he and I call the "carabiner incident", but he has regarded me suspiciously ever since.
If the elephant ever does forget, the crow will be there to remind him.