Hispanics now account for more than 15% of the U.S. population, and their surge is largely the result of births among people already in the country, according to new Census Bureau data.

In an annual report, the Census said there are 45.5 million Hispanics in the U.S., up from 35.7 million in 2000, when they made up 12.6% of the population. It said growth among Hispanics was responsible for half of the U.S. population gains between 2000 and 2007.

In...