I've personally witnessed Sea Gulls carrying rocks to smash shellfish as well as carrying shellfish to smash them into rocks.
Odd question: Are there any birds that would carry rocks in flight?
African swallows carry whole coconuts.
Quite apart from the answers already given...
It's common for berry-eating birds to carry seeds with them, which they can drop with the accuracy of a bomb-sight. But there are other things, too.
I was standing in a city park watching a chess game being played with several dozen kibbitzers when a high flying pigeon decided it was potty time. It got the guy next to me right on the head. Horrible mess. The poor guy had no sense of humour at all, but the rest of us had a good laugh.