"In Gilbert’s experience, the trick works better on cherries than apples, namely Honeycrisp. Starlings peck through the apple skin and eat the fruit from the inside. Gilbert doesn’t use sugar on wine grapes because they don’t get washed before processing and the extra sugar would affect fermentation."
That's what I was thinking. I like the idea, but I think you'll end up with a few more degrees brix than you intended.
As for Concords grapes, they don’t have a starling problem, said Mike Concienne of the National Grape Cooperative, which owns the Welch’s brand. Concords naturally produce methyl anthranilate, the active ingredient for many chemical bird repellents used on corn, rice and golf courses.
I didn't know this. I know some other people in the area and the birds seem to have no problems going after them. Perhaps it is only the starlings that don't like the Concords.