i've only seen that behavior in person once, and it was so far away i wasn't able to get any decent photos. but it's fairly common, especially among juveniles. adults do it too, though for them it's more about rivalry for territory or mates. usually it's about the equivalent of two guys in a tavern arm-wrestling for beers, and the two combatants let go and fly off, but there have been cases where the birds remain locked together and fall to their deaths