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 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 amassed enough savings in the land of opportunity 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.