You have sacrificed at least three artifacts and/or creatures this turn, plus an additional artifact or creature for each time you've cast Dargo from the command zone this game.
({R} magic symbol) available.
Steps
Cast Dargo from the command zone by paying ({R} magic symbol) .
Birgi triggers, adding ({R} magic symbol) .
Activate Carrion Feeder by sacrificing Dargo.
Choose to return Dargo to the command zone from your graveyard.
Resolve the Carrion Feeder ability, putting a +1/+1 counter on it.