PARLIER - For the past century, raisins in California's Central Valley have been harvested in exactly the same way; a monthlong frenzy of hand picking that required more workers than almost any other crop.Last season, many raisin growers turned to machines to do the work. Although they had long held out, they are now joining growers nationwide in embracing mechanization to fend off global competition.But the switch to mechanical harvesting is taking a heavy toll on the Mexican migrants who fill most of the state's lowest-paying farm jobs. With machines picking more crops, the need for field hands is falling...