ZHUZHOU, China - Regulars of the Jianba barbershop in this southern city recently found it shuttered, with a curious note taped to the door.

Dear customers, I got a call from my daughter yesterday. I have been away from her so long she doesn't even know how to call me 'Daddy' anymore. . . . I beg you for a week off to visit my family.

The letter, photographed by a passerby, was posted on the Chinese equivalent of Twitter and quickly went viral. It reflected a growing angst in this country over "left behind children."

About one-fifth of China's children - more than 61 million - live in villages without their parents. Most are the offspring of peasants who have flocked to cities in one of the largest migrations in human history. For three decades, the migrants' cheap labor has fueled China's rise as an economic juggernaut. But the city workers are so squeezed by high costs and long hours that many send their children to live with elderly relatives in the countryside.

The barber who posted the note, Wu Hongwei, and his wife had left their daughter with her grandparents in a remote village when she was 9 months old. The couple thought the 340-mile distance wouldn't be a barrier.

Every day, they phoned and told the little girl, "Mommy loves you" and "Daddy misses you." They taped photos of themselves on the concrete walls of her room at her grandparents' house.

But after almost two years, they have come to a stark realization.

"We are complete strangers to her," Wu said.

Wu, 24, left the tiny village of Zhaishi in the craggy mountains of Hunan province eight years ago. Staying would have meant back-breaking labor for just $3 a day - when work could be found.

He bought a bus ticket to Zhangzhou, where his uncle took him on as an unpaid barber's apprentice. Wu then moved to Zhuzhou, where he got a job pulling in $500 a month.

It was in that city that Wu met Wang Yuan, a woman with an infectious laugh. The barber wooed her with his guitar and folk songs. And, for a while, life in the city seemed full of possibilities for the newlyweds. They had their daughter, Beibei, in 2011. To take care of the baby, Wang, 33, quit her job selling cellphones. Her husband worked extra hours, cutting hair from morning to 11 p.m.

At first, they managed to get by. They kept up the $100-a-month rent. Like many migrant workers, Wu had to help support his peasant parents; $170 a month went to them.

But then their baby was weaned and needed formula, an expensive product in China, where parents distrust cheap local brands that often turn out to be tainted. "There was no choice. We both needed to keep working," said Wang, whose parents were too sick to help.

So, in May 2012, the couple made the journey to Wu's village and gave the baby to his parents. The first few months were excruciating. "I went to sleep hugging the little outfits she left behind," Wang said. "I cried constantly."

Three months after dropping off their daughter, the couple returned to the village, eager to visit. As soon as they walked through the door, Beibei hid from them. "Whenever we tried to hug her, she screamed and clung to Grandma," Wang recalled.

In recent years, the plight of "left-behind children" has attracted growing attention. Chinese experts warn of psychological and emotional problems for kids raised apart from their parents. Such children often do worse at school than their peers. Studies have suggested they develop increased tendencies toward suicide and alcohol abuse.

But in cities, migrant children face problems, as well; they are often barred from public schools and medical care unless their parents have residency permits.

Wang and Wu have started planning to bring their daughter to the city permanently. They have set a deadline: the beginning of February, after the Chinese New Year.