A few years ago, Mr Adalberto Martin began to see some troubling changes at work.

As a veteran of the room service staff at Marriott's W Hotel in downtown San Francisco, he was an expert in delivering carefully assembled trays of food and drink to hungry guests. But the number of orders had sharply decreased. What was once 50 glasses of orange juice every morning had dwindled to 10, and Mr Martin's tip income fell accordingly.