That would certainly be a very frustrating situation no doubt.... I think you're right though, there have to be other places where they are also roosting at the end of the day, maybe even places where the land owners would be receptive to you shooting them. Take a drive or a walk and see if you can find some of these other areas. Better yet,,,, I'd go talk to the owners of the vineyard where these birds are feasting all day on grapes. I guarantee you that the vineyard owners are not to pleased about having thousands of yellow beaks in their grape vines all day eating their crop...... maybe they'll give you permission to help keep the birds away...... The owners of the duck farm I pest hunt at have thanked me many times for helping to control the woodchuck, starling and pigeon problems they have each year.
Jeff