A few weeks ago there was a post about Crowded Cities, a startup trying to train crows to pick up and throw away cigarette butts in the city for a food reward. I'm sure it'd be harder, but could crows also be incentivized to somehow pick up rats?
Quick googling suggests a rat is roughly half the weight of a crow - so I imagine it would be somewhat difficult for the crow to pick up anything other than baby rats.
It would be amazing if they could get the crows to collaborate, e.g. two crows carrying away a rat. Probably impractical but I love the idea.