I can't take credit for the idea. I heard it on a "You Bet Your Garden" podcast. If you feed the birds less during the warmer months, they'll have to actually go looking for food.
Usually my roses are done with their spring flush of blooms by the time the JBs show up. DD in MI gets swarms of them on her raspberries. I didn't have as many JBs this year - only a couple at a time spread out over 8 weeks. I did cut down my porcelain berry vine which seems to be a favorite for them. Not sure if that made the difference or not.
