PHOENIX (Reuters) - For the first time births have overtaken immigration as the main driver of dynamic growth in the U.S. Hispanic population, according to a new study released on Thursday The Pew Hispanic Center survey found the new trend especially evident among Mexican-Americans, who account for almost two thirds of the U.S. Latino population. The study, which drew on U.S. Census Bureau data and other government sources, noted the Mexican-American population grew by 7.2 million as a result of births in the decade to 2010, while new immigrants added 4.2 million people. ... The study found growth in the...