LOS ANGELES — 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 undocumented immigrants from Mexico, for a decade at her Quail Ridge Dairy in Colorado.
Then about a year ago, the brothers informed Ms. 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 undocumented population to its lowest level in more than 15 years.
New data that will be released on 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 Trump has stepped up deportations across the interior of the United States and further fortified the southwestern border against unauthorized entry.
Several Democratic candidates have called for decriminalizing border crossings; establishing pathways to citizenship for undocumented immigrants brought to the United States as children; and relying on technology, not more fencing, to enforce the border with Mexico. They have also expressed support for focusing deportation resources on removing immigrants who are a threat to public safety or convicted criminals.
The new data shows that the number of undocumented immigrants continues to shrink, a trend that began even before Mr. Trump took office.
The population of unauthorized Mexicans in the United States declined by a quarter between 2010 and 2018, the new immigration figures show, amid stepped-up deportations and an improved Mexican economy that has encouraged many people to go home voluntarily.
And Mexicans, the largest foreign-born population in the United States, are not the only nationality electing to leave. The undocumented population from South Korea has dropped by 22 percent, and Poland’s has plummeted more than 50 percent — 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.”
Mexico’s gross domestic product is now larger than that of Canada’s. Its birthrate has dropped, meaning families have fewer mouths to feed, and getting across the increasingly fortified border with the United States has become more difficult, dangerous and expensive.
“It used to be that a glut of working-age Mexican citizens turned to the U.S., a developed country, for available jobs,” said Donald Kerwin, executive director of the Center for Migration Studies, which is based in New York and analyzes trends and policies related to international migration. “This dynamic has changed significantly over the years.”
The center’s estimates show that the overall number of undocumented people has dropped precipitously in California, New York and New Jersey, states that for decades have been magnets for unauthorized workers and that, in recent years, have introduced sanctuary policies to protect them.
Texas, whose governor and Legislature have backed efforts by the Trump administration to crack down on illegal immigration, has experienced an increase in its undocumented population, suggesting that the job opportunities and affordable living luring Americans to the state are also wooing undocumented immigrants.
California’s undocumented population dropped to 2.3 million, a 21 percent decline since 2010. New York saw a 25 percent decline, to 684,000. Texas’ undocumented population climbed to 1.79 million from 1.71 million.
Despite the arrival at the border of a large number of Central Americans, especially families fleeing violence, a larger proportion of undocumented people who have arrived in recent years came on visas that they then overstayed.
About 4 million of the 10.6 million undocumented immigrants who 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. Many of them hail from Asian countries, such as China and India.
There was a 69 percent jump since 2010 in the number of Indians in the country illegally, reaching 619,000 in 2018. The number of undocumented Venezuelans more than doubled during that period, driven by political and economic upheaval.
Conversely, Ecuador was one of the nationalities seeing the biggest declines. The number of undocumented Ecuadoreans in the United States shrunk by 36 percent, leaving 173,000 people. Among those who went home was a 50-year-old construction designer, Mario, who left five years ago after living for 14 years in New York.
“My main goal was the education of my daughter and my son,” said Mario, who withheld his last name out of concern for undocumented relatives still living in the United States. “Both of them, they got college degrees. I accomplished my mission.”
Not everyone is returning by choice, of course.
After dropping to 65,332 in the last year of the Obama administration, deportations of people from the interior of the country have climbed, reaching 85,958 in the most recent fiscal year.
The Department of Homeland Security recently announced it intends to deploy SWAT-like teams of border agents to help arrest immigrants who live in so-called sanctuary cities like Chicago, a plan likely to bolster that number.
The Trump administration has also limited some previously available exemptions for people fighting deportation.
Jorge Zaldivar of Mexico, for example, had been allowed under the Obama administration to stay in the United States despite a deportation order because he had an American son with a congenital illness who needed his support.
But Immigration and Customs Enforcement refused to extend his stay of deportation last year, and Mr. Zaldivar was deported last month even though he still has a case before an appeals court.
Now his wife, Christina, an American who barely speaks Spanish, is preparing to move from Denver to Mexico with their three youngest children.
“I have to sell the house and give up everything we worked hard for,” Ms. Zaldivar said.
The diminishing number of undocumented immigrants is becoming an ever-greater concern for employers in sectors of the economy, such as agriculture, that rely on immigrant labor.
“You invest in developing these people who become a huge part of your operation, and then they’re gone,” said Ms. Kraft, whose family-owned dairy in Colorado produces milk to make cheese, chocolate and whey powder.
“You lose that historical knowledge and have to start out with new ones,” she said, “except new ones aren’t coming to replace them.”