But that means we can't call dusk shields ducks anymore.
D:
Since this is a humor card, you don't have to take any of the following seriously if you don't want to. I think this could be submitted as a card. I like it
But before that, I have a few suggestions.
What you do for the duck, is below the table for Duck Shield you add another table for Duck | Swan - that's where its casting cost and stats go.
For the text:
"Shield: If you received 7 or greater damage on the opponent's turn, spawn a Duck | Swan."
OR
"Shield: Whenever you take 7 or more damage, spawn a Duck | Swan."
(I included the part about the opponent in case you don't want this to be trigger-able by bolting yourself. Or trigger by poison.)
The way it is, you're practically guaranteed a free Abyss Crawler every (other) turn. It's like Mitosis with an hp cost. Probably lower the cost for both upped and unupped by 2 or so.