From the accompanying article, Far Fewer Enter U.S. Illegally From Mexico:
Arrests of people trying to sneak into the U.S. from Mexico have plunged to the lowest level in four decades, the latest sign that illegal immigration is on the retreat even as legislatures, Congress and presidential candidates hotly debate the issue.
Behind the historic drop is a steep decline in the birthrate in Mexico and greater opportunities there relative to the weak U.S. economy. Stepped-up U.S. patrols along the border make it both riskier and more expensive for Mexicans to attempt to enter the country.
Government crackdowns on U.S. employers who hire illegal workers also have discouraged immigrants.