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– José cared for the bottle-fed babies, 700 of them in all. He knew a calf was healthy if her eyes were bright and her appetite hearty. Droopy ears were a bad sign. He was attuned to calf coughs.

“His job was to do all things a mom would do to look after her young,” said Mary Kraft, who employed José and his brother, Juan, both immigrants from Mexico in the country illegally, for a decade at her Quail Ridge Dairy in Colorado.

Then about a year ago, the brothers informed Kraft that they were returning to Mexico. They had milked the land of opportunity and amassed enough savings to resume their lives back where they had started.

The pair are among a growing number of Mexicans who have been departing the United States in recent years, part of a reverse migration that has helped push the population of people in the country illegally to its lowest level in more than 15 years.

New data that was released Wednesday by the Center for Migration Studies shows there were 10.6 million immigrants living unlawfully in the United States in 2018 compared with 11.75 million in 2010, a decline propelled primarily by Mexicans returning south.

The issue of illegal immigration has become a centerpiece of the 2020 presidential campaign, as President Donald Trump has stepped up deportations across the interior of the United States and further fortified the southwestern border against unauthorized entry.

The new data shows that the number of immigrants in the country illegally continues to shrink, a trend that began even before Trump took office.

The population of Mexicans in the United States illegally declined by a quarter between 2010 and 2018, the new immigration figures show, amid stepped-up deportations and an improved Mexican economy.

And Mexicans, the largest foreign-born population in the United States, are not the only nationality electing to leave. The population in the U.S. illegally from South Korea has dropped by 22%, and Poland’s has plummeted more than 50% — returning to countries that have enjoyed economic prosperity.

“It is widely assumed that everyone wants to come to the United States but that no one wants to leave,” said Robert Warren, the demographer who analyzed census data for the nonpartisan think tank. “That’s never been the case.”

About 4 million of the 10.6 million immigrants who illegally resided in the United States in 2018 arrived after 2010. Among them, two-thirds, or 2.6 million, entered the country lawfully, having passed inspection at an airport or another port of entry, but did not leave within the period of time they were permitted to stay with a tourist, business or student visa.