Birds might begin feasting on the grapes before they ripen, but they will quickly decimate the bunches after the grapes are ripe. Harvesting as soon as the grapes are ripe gets them away from the birds. Grapes develop their full mature color of green, purple or red early, depending on the variety, which may fool the birds into feeding early. Grapes are fully mature when they develop a dull whitish, almost powdery, bloom on the fruits. Picking them at this point can save some of the grapes from the birds, but you may need to use other methods if the birds invade the grapevines before the grapes are ripe enough to harvest.
Paper bags protect grapes after they have begun to develop so they have time to reach maturity before becoming bird food. Use paper bags larger than the mature size of the grape clusters. Each cluster requires its own bag, so this method works best if you only have a few grapevines. Slide the bag over the cluster when the fruits are halfway to full maturity, or after the clusters begin to form but before the grapes reach full size or color. Secure the neck of the bag closed around the vine, supporting the grape cluster using twine or a twist tie. The bags also protect the developing grapes from moisture damage during the last stages of ripening.
Although more expensive, bird netting provides a simple option, especially if you have several grapevines. Draping the netting over the grapevines protects some of the clusters, but the birds can still reach some of the grapes through the netting. Installing stakes or posts taller than the plants at either end of the row, and then draping the netting over the posts, elevates the netting so the birds can't reach the grapes. Anchor the bottom of the netting to the ground with rocks so small birds don't hop under the net to reach the fruits.
Scare devices use sound or movement to frighten birds away. Chemical repellents are also available, but they have limited usefulness in a home garden and they may increase activity from other pests, such as Japanese beetles. Distress call sound devices work well in a backyard, but they must be programmed for the specific bird species. Visual devices are the simplest to use. Holographic bird tape tied to stakes blows in the wind and flashes in the sunlight, frightening birds. Mechanical devices, such as motion-activated sprinklers or scarecrows, also work well. Birds do become accustomed to scare devices, so change the type of scare device each year to keep the grapes safe.