The ducks are blatantly obvious. Duck #1, on page 9, is a “furniture” duck, part of the physical scene of the story. The ducks at the top of page 15 are “atmospheric” ducks—part of a calm sylvan scene where Touko thinks how she’s never been so calm when someone has confessed to her. The calm exterior scene mirrors Touko’s interior calm. You potentially could think of that as “symbolic,” but it’s really just parallelism. (If you start to doze on a quiet beach, the sounds of the waves aren’t “symbols” of your relaxation, they’re part of it.)
But then Touko notices the pair of ducks floating nearby, and their ducky skinship (beakship?) suddenly reminds her of her previous close relationship with Yuu. The connection is more associative than technically symbolic—person sees X, which reminds her of Y. Touko’s demeanor changes, which Sayaka notices, then we cut away from the scene.
Then the ducks reappear at the crucial moment, after Touko has announced her decision and just before Sayaka confirms that Yuu is the one Touko has chosen. Then as Sayaka reflects on what has happened, she sees a duck high in the sky, watches it land beside another duck, then sees them as a couple in the distance while she thinks “It didn’t end up being me.” Sayaka, razor-sharp as she is, most likely explicitly makes the connection between Touko’s previous reaction to the ducks and her relationship with Yuu. (I am very fond of Sayaka in this moment and sad for her, but am glad the ducks have found contentment).
This panel might as well have fucking neon signs saying, “This duck represents Touko,” and “This duck represents Yuu” and “This panel represents how Sayaka is a proud but sad fucking bunny rabbit.” (I just made up the part about the bunny rabbit.)
