Daulah, Aerial Merchant

At the beginning of each opponent's upkeep,
that player may choose a nontoken permanent
he or she controls and a permanent you control.
You may exchange control of the chosen
permanents. If you do, you and that opponent
each create a 0/1 green Bird creature token with
flying and "{t}: Add one mana of any color."
"One thing Burren hasn't figured out yet is how to
halt trade on the wing."