Mexican immigrant Bernardo Ortega Guerrero of Vera Cruz, Mexico, rides in a U.S. Immigration and Customs Enforcement bus while heading to a waiting deportation jet in Broadview, Ill., in May. Experts say increased enforcement may be one factor for the decline in the unauthorized immigrant population in the U.S.

The Pew Hispanic Center says the number of unauthorized immigrants in the U.S. fell 8 percent between 2007 and 2009. The largest decline occurred in the South Atlantic and the Mountain West. The report, released Wednesday, does not specify causes, but experts point to the economy and increased enforcement as possible explanations.

For several years, and certainly since the recession began, the number of immigrants entering the U.S. illegally has been dropping sharply.

Now, evidence from the Pew Hispanic Center says that more are also leaving the country.

In a new report, Pew finds that the number of unauthorized immigrants fell 8 percent between 2007 and 2009, the biggest such decline in two decades.

The change is dramatic. During the first half of this decade, an estimated 850,000 people came into the U.S. illegally each year. The Pew Hispanic Center says that as of last year, that number is believed to be down to 300,000 and still dropping. Yet until recently, even as the recession deterred newcomers, many immigrants already here insisted they would ride out the downturn, and there was little evidence of an exodus. No more.

In his latest study, based on a detailed analysis of census figures, Pew demographer Jeff Passel says the biggest outflow was in the southeast.

"Our estimates show significant drops in Florida and Virginia," he says. "But basically, [across] the whole South Atlantic coast, from Delaware down to Florida, we show significant decreases."

The Mountain West also saw a drop in unauthorized immigrants. The largest was in Nevada, and Passel says Arizona, Colorado and Utah combined saw a sizable decrease. No state saw a significant increase in unauthorized immigrants.

Passel says those leaving are mainly from the Caribbean and Central and South America -- but not Mexico.

The Pew report does not look at why immigrants may be leaving, but evidence points to the terrible economy as one reason. In fact, Pew finds that the unemployment rate for unauthorized foreign workers has risen to 10.4 percent, and that represents another reversal.

"In the middle of the decade," Passel says, "when the U.S. economy was relatively strong, unauthorized immigrants actually had lower unemployment rates than U.S. natives or legal immigrants."

But people on both sides of the immigration debate also point to stepped-up enforcement.

"We're seeing widespread sweeps by federal and local law enforcement officials. They stretch across our state," says Cheryl Little, executive director of the Florida Immigrant Advocacy Center. Pew found that state's undocumented population down by 375,000 -- a whopping one-third of the total.

"When we visit the detention centers," Little says, "they're often filled with immigrants who were stopped by local police for routine traffic violations and then turned over to [federal Immigration and Customs Enforcement]."

ICE is now deporting some 400,000 immigrants a year, double the number just a few years back.

There has also been mounting anecdotal evidence in recent years that some immigrants, fearful of arrest, are deciding to leave the U.S. on their own.

Of course, counting unauthorized immigrants is hard. No one knows an exact number, but the Pew estimates are widely regarded as trustworthy. Nationwide, the Pew report finds the total number of illegal immigrants is now down to just over 11 million. Still, the report notes, that's a third more than were in the U.S. a decade ago.

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