Maybe they're outside in the garden. They could be playing softball. Or perhaps they're just plain bored.

In TV's worst spring in recent memory, an alarming number of Americans drifted away from television the last two months: More than 2.5 million fewer people were watching ABC, CBS, NBC and Fox than at the same time last year, statistics show.

Everyone has a theory to explain the plummeting ratings: early Daylight Savings Time, more reruns, bad shows, more shows being recorded or downloaded or streamed.

Scariest of all for the networks, however, is the idea that many people are now making their own television schedules. The industry isn't fully equipped to keep track of them, and as a result, the networks are scrambling to hold on to the nearly $8.8 billion they collected during last spring's ad-buying season.

"This may be the spring where we see a radical shift in the way the culture thinks of watching TV," said Sarah Bunting, cofounder of the Web site Television Without Pity.

The viewer plunge couldn't have come at a worse time for the networks - next week they will showcase their fall schedules to advertisers in the annual "up front" presentations.

The networks argue that viewership is changing, not necessarily declining. Some advertisers respond that they are no longer willing to pay full price up front to reach viewers who may not tune in later.

This fall, both sides will be watching what happens with families like Tony Cort's. During prime time, Cort, his wife and four kids tend to scatter to computers or other activities in different parts of their New Jersey home. (Not during American Idol or Lost, though.) They're definitely watching less TV, said Cort, who runs a Web site for martial arts aficionados.

"I remember when 24 was on, that was something there was a lot of interest and excitement about," he said.

News flash: 24 is still on. Its ratings are down, too, amid a critically savaged season.

In the six weeks after Daylight Savings Time started in early March, prime-time viewership for the four biggest broadcast networks was down to 37.6 million people, from 40.3 million during the same period in 2006, according to Nielsen Media Research.

More bad news abounds. NBC set a record last month for its least-watched week during the last 20 years, then broke it a week later. This is the least-popular season ever for CBS's Survivor. ABC's Lost has lost nearly half its live audience - more than 10 million people - from the days when it was a sensation. The Sopranos is ending on HBO, and the response is a collective yawn.

For its first four episodes this season, The Sopranos averaged 7.4 million viewers for its weekly Sunday night premiere, down from 8.9 million at the same point last season. But HBO shows each new episode eight times a week. Between the multiple plays and DVR viewing, each episode this spring gets 11.1 million viewers, down from 13 million last year. And these figures don't count people who watch on demand.

Numbers for The Sopranos may be down because people can watch whenever they want. They may not be as interested in the show as they used to be - or it could be a combination of both.

Television has made billions based on how many people watch a show at its regular time.

That idea may already be obsolete. So should the industry use DVR viewing when setting ad rates? If so, how quickly must people watch the shows - within two days? A week? What about people who watch shows on their cell phones or on network Web sites, which Nielsen doesn't measure yet? Later this month, Nielsen will begin measuring how many people watch commercials. Should those be used to compute advertising costs?

Right now, none of those questions have answers.

However, "if we continue to do business assuming people will watch television as they always have," said Alan Wurtzel, NBC's chief research executive, "it's a dead-end game."