Most work at least 28 days a month; between 50 and 70 percent of them will send their pay, called remittances, back to the family. The money is used to help build a house, to get aging parents off the farm, to make sure that a brother can continue to study at local schools that lately require tuition, or to finance a small shop.
"Without migrants, the whole structure would collapse," says David Wank, a China specialist at Sophia University in Tokyo. "They staff the shops, restaurants, factories, construction. It all depends on migrants."
Interviews with dozens of migrants demonstrate their great resilience. In the social strata they are an often invisible if not forgotten sector whose capacity for cheap labor and long hours are matched by their ability to face rugged obstacles such as crowded workplaces, police detentions or abuse, and avaricious employers who often withhold pay or otherwise exploit them, knowing they are easily replaceable. NGO groups that organize unofficially to help migrants often work under great pressure, their leaders say.
Despite low status, migratory workers represent a real change in China. For the first time in this ancient place a significant number of peasants are leaving rural areas where their families have lived for eons. Many will return to settle in the village - but an increasing number plan not to, as land and jobs in the villages remain scarce.
At the same time, migrant earnings from the east partly help to steady China's economy as it moves from farm to city. Their remittances, sent back to China's interior regions, help ease the poverty as countless farm villages continue to struggle and to be "hollowed out," as a Chinese rural economist Li Chang Ping puts it.
"The migrant remittances are crucial to stabilizing the countryside, while the central government figures out how it will handle the rural issue," says one Beijing scholar.
Migrants have limited legal standing. No union represents them, and for the scant number of Chinese that contemplate organizing, the millions of new migrants are a "worst case scenario," as one expert puts it.