Our largest thrush the American Robin is found throughout North America and Canada. Large flocks form and roam over the countryside and as their breeding season diet of insects diminishes they turn their attention to berries, and unfortunately for grape growers, grapes. The birds will peck at the grapes to open them up and this is a greater hazard than if they simply ate a few grapes. The now damaged grapes are succeptable to fruit fly infestation and the entire cluster of grapes is vulnerable to mold.
Robins are territorial and will take up residence and even when shot at, the vast majority of the flock will remain.
Netting is the most effective way of addressing a robins problems. Every third row netting is installed from waist height down. The birds which tend to hop along the ground find the netting claustrophobic and tend to leave the netted area.