Why would you want to punch a bird in GTA? Because it's both kind of difficult to do, and because it's funny, that's why.
Bird punching isn't the only thing that DefendTheHouse discovers is possible in GTA V in this mythbuster video. You can, for example, land a fighter jet upside down... assuming you have the skill to do so, of course!
Grand Theft Auto V Mythbusters: Episode 10 [DefendTheHouse]