"Diese sind die Augen einer Ente" would work, too, right? "Die sind die Augen einer Ente" would mean "Those are the eyes of a duck," oder?
das can not be a singular and why it says das sind? It should be diese sind
"When it is used with the verb sein, the form das can refer to singular and plural nouns of all three genders"
That article went over my head, but thanks for the explanation.
Why not "That's the eyes of a duck"?
Because "sind" is not a singular verb.