This is my version of why Jace is afraid or hates ducks. Enjoy
Jonathon sat in his room, drapes closed, and placed the duck in the center of makeshift altar. The altar was made form his schoolbooks with a black sheet concealing them. Drawn in the center of this altar in white crayon was a pentagram in which the duck stood. It quacked, but was not able to move from it.
"Yes, I think this is going to work." Jonathon whispered to himself. He was all alone in the house; his father, Valentine, was away doing what he called business. Jonathon didn't mind it, at least not today. This way he could perform his ritual without his father knowing.
He knew that his father was seeing another "son". Jonathon had never seen this "son", but he could tell. Sometimes Valentine came back from his "business" trip with a smile. It was simple but Jonathon could see beyond it. It wasn't just a simple smile; it was a smile that was proud. That was the smile he got when he hit his target right, it was a smile a father gave his son when he was happy with him. Now this was only what brought up the suspicion, but Jonathon had proof. There was the time. This time was when Valentine came home with a suspicious object. The suspicious object had fallen out of Valentine's pocket and Jonathon scooped it up before his father could notice. It appeared to be some kind of card, but nothing like a playing or tarot card. No, this was different. All it had on it was a man in a baseball uniform. Jonathon could see no purpose this served, but wondered why, if he was, this man was important enough to be pictured on a card. At the age of nine Jonathon figured he had a pretty good understanding of the world that he lived in, but this had to be a mundane thing. The only reason he could guess would be an exploitation of this player to fans of the sport. Jonathon thought on. The general demographic for this type of thing were young boys. On this thought he deduced that this useless card came from none other that the other "son". An idea then formed in his head.
Jonathon never knew if his father knew it was there, because he never looked for it. Maybe, Jonathon had thought, the boy had put it there without Valentine noticing. Either way this was all he needed. He not only had proof, but also had a means of carrying out his ritual.
Jonathon now pulled out the card and placed it on his altar. Next, he pulled out a curse book he found in his father's library and opened it to the marked page. There on the page ancient lettering was scrawled out circling in and around like a snake. He ran his hand over the page feeling the welt of the ink. Jonathon couldn't be sure if it would work, but he wanted to try. He began to read with a voice clear and steady. As he read Jonathon propped the book on his altar and sat a silver goblet in front of him. The goblet was old and plain with a wide rim and ancient carvings. Jonathon picked up his hunting knife and ran the blade across his left hand. His dark tainted blood trickled down into the goblet and as the last words left his mouth the blood began to swirl. The curse spell (part one) was working. It took his blood and mixed and multiplied it. Jonathon watched, as the blood became ticker and blacker until it stopped just a millimeter away from the rim. A smile stretched across his face, his eyes sparkling, and he picked up the goblet and carefully placed it to the right of the duck.
Part one was finished. Now on his altar there were the three ingredients to carry out his spectacular plan. Jonathon bandaged his hand so that blood wouldn't get anywhere else and referred to his book for part two. This part was the enchanting spell. Jonathon took a breath and began. As he chanted he picked up the baseball card and dipped it into the goblet. The blood swirled around the card and then completely covered it taking the card until it disappeared. He dipped his own figure into the goblet and smeared the blood over the duck's head, back, and feet. It was finished. The curse spell seemed to be taking affect on the duck. This time Jonathon was almost giddy with excitement but remained as calm and collected as his father taught him. Now time for the third part. This part was much more difficult than the others as the spell could only be performed by warlock. Jonathon had found this inconvenient, but doable. He knew a warlock or two and was able to slyly, but not easily, convince one to recite the spell. He recorded it secretly.
Jonathon shut the curse book and placed it on his bed so that nothing, if something went wrong, would harm the book. Of course Jonathon couldn't even be sure it was the right spell, but he had to try. Jonathon went and sat in front of his altar once again, took out the recorder, placed it next to the duck, and pressed play. The spell sounded filling the silence Jonathon created. He wanted to make sure nothing could mess up the spell. He kept his eyes on the duck and finally he saw the spell taking shape.
The duck began to quack and scream until there was two of them, and then it started again. From two to four to eight the ducks kept multiplying quacks filling the room. Jonathon stood amongst his multiplying ducks and laughed. "It worked!" He yelled, "GO FORTH MY ARMY OF DUCKS AND MULTIPLY! GO FORTH!" Jonathon lead the ducks through the house to the front door. The pitter-patter of duck feet and quacks sounded throughout the house until the very last duck left the threshold. Jonathon smiled. Now that boy will be ridiculed for his ridiculous fear of ducks.