Migrant workers who once crossed the border each spring from Mexico and returned south for the holidays at home are more likely than ever to spend Christmas in the United States. It is a sea change in a once predictable migratory flow - and an unintended consequence of U.S. immigration policy that aims to keep people out by investing billions of dollars in beefier border security. The new reality is clear in Northern California's Napa Valley, where farmhands once packed for Mexico after harvesting prized wine grapes. The vines are now bare, but many workers are still kicking around for...