Some gardeners use scarecrows, but before long they become roosts for neighborhood birds.
Years ago we strung up a scaring device to keep birds away from our blueberries. Not only was it ineffective, but the birds actually used it for a roost upon which to perch before swooping down onto the ripening fruit. The effectiveness of the newer ultrasonic devices is as yet not well-proven, and the devices can be fairly expensive. But the garden should be a peaceful place, not a zoo, and all of these dazzling, noisy contraptions are not only not particularly effective, they are downright annoying. The most effective device for preventing bird damage is plastic netting.