This week, the television upfronts  in which the broadcast networks present their schedules to advertisers  will open with a mystery. Who stole six million viewers?

That’s the number who were watching prime time television last May, a month affectionately known as “sweeps,” but have disappeared this year, according to the overnight Nielsen ratings. Each of the major broadcast networks, save for Fox, has seen its audience decline this season. The ratings for hit shows like “American Idol” and “CSI” have approached record lows.

Where some of last May’s 44 million viewers went is not a mystery, according to the networks. The writers’ strike this winter deflated the ratings and accelerated the flight of viewers to cable channels.

But the more significant shift can’t be blamed on the strike. In the past television season, there has been a sharp increase in time-shifting. Some of the six million are still watching, but on their own terms, thanks to TiVos and other digital video recorders, streaming video on the Internet, and cable video on demand offerings. So while overall usage of television is steady, the linear broadcasts favored by advertisers are in decline.