"You appraise the parrots."
Translation:Tu psittacos aestimas.
psittaci is a nominative form (if the parrots were doing the action this would be used).
psittaci aestimant -> 'the parrots appraise'
You need to use psittacos since that is the accusative (used for what the action is being to).
psittacos aestimant -> 'they appraise the parrots'