They poop on people they don't like. I used to work at an office where you had to walk under some trees from the parking lot to the entrance. There was one guy that got pooped on at least once a week. Then he used an umbrella which they pooped on too. No idea what he had done to them but they clearly didn't like him :-)

Another thing I saw at a park: The crows surrounded a swan. One would pick at the swan's butt. When the swan turned around, another would do it. It looked like they bullied the swan for entertainment.