The only option is physically protected the fields, i.e. enclosed the area with bird-friendly nets. They will solve the problem completely but most farmers do not like it due to the cost problem.
Hanging bird deterrent agents, e.g. hanging plastic falcons and a ballon with big eye as pattern, could reduce the severity of the problem, but cannot solve the problem.
Alternatively, we could play calls of raptors around the field regularly. It may scare the birds but they may use to it subsequently.
There are plenty of bird deterrent agents on the web. (available to order online). Google search may help.
