In fact, we think that, despite the size of the decks, learning how to play Commander is the best starting point for people looking to play MTG with more players
(8/7/2020) Permanents put onto the battlefield due to Karn's ability will have been under the starting controller's control continuously since the beginning of that player's first turn
Confirming the card number and treatment before looking at any price removes most of that error
It's super flexible, as you can use it when your own horde swings in, or when another player goes in on the offensive (they don't even have to be sending everything/anything at you.) However, while I can see this card working pretty well in any blue/white/green tokens deck, it's obviously best with an MTG commander that cares about birds
If, during your declare attackers step, Ursine Monstrosity is tapped, is affected by a spell or ability that says it can't attack, or hasn't been under your control continuously since your turn began (and doesn't have haste), then it doesn't attack