NPR's Jim Zarroli reports that day-labor markets have become commonplace in America's big cities. Immigrant laborers looking for a job for the day, or maybe for the week, begin arriving early in the morning at a particular street corner. Often they congregate in the same spot day after day, year after year. These men make up an important underground work force, especially in the construction and building trades. A new study says many prefer day work to steady employment because day jobs pay better.