WASHINGTON -- A record number of U.S. counties -- more than 1 in 3 -- are now dying off, hit by an aging population and weakened local economies that are spurring young adults to seek jobs and have families elsewhere.New 2012 census estimates released Thursday highlight the population shifts as the U.S. encounters its most sluggish growth levels since the Great Depression.The findings also reflect the increasing economic importance of foreign-born residents as the U.S. ponders an overhaul of a 1965 federal immigration law.Without new immigrants, metropolitan areas such as New York, Chicago, Detroit, Pittsburgh and St. Louis would have posted flat or negative population growth in the last year."Immigrants are innovators, entrepreneurs. They're making things happen. They create jobs," said Michigan Gov. Rick Snyder, a Republican, at an immigration conference in his state last week.Census data show that 1,135 of the nation's 3,143 counties are experiencing "natural decrease," where deaths exceed births.That's up from roughly 880 U.S. counties, or 1 in 4, in 2009. Already apparent in Japan and much of Europe, natural decrease is increasingly evident in large swaths of the U.S.Zones of natural decrease stretch from industrial areas near Pittsburgh and Cleveland to the vineyards outside San Francisco to the rural areas of East Texas and the Great Plains.A common theme is a waning local economy.In contrast, the Texas metropolitan areas of Dallas, Houston and Austin continued to be big draws for young adults, ranking first, second and fourth among large metro areas in domestic migration. Analysts credit diversified economies that include oil and gas production.Phoenix, Las Vegas and Orlando, Fla., also saw gains.