Reminds me somewhat of this project, which trains crows to trade objects (coins, in this implementation) for food. I've been meaning to build something similar at some point.
I haven't decided what to get them to do for the food. Cash isn't used much here, so they'll have a hard time finding quarters. I was thinking pop or beer bottle lids, or something similar to that, would be good. That way they trade an abundant trash source for food, and clean up at the same time.
Though I suppose it could lead to the unintended effect of them getting into dumpsters to cheat the system, if they're smart enough...
I think ravens are supposed to be even more intelligent?