Across the television landscape, network and cable, public television and pay cable, English-language and Spanish, viewing for all sorts of prime-time programming is down this spring — chiefly among the most important audience for the business, younger adults.

In the four television weeks starting March 19, NBC lost an average of 59,000 viewers (about 3 percent) in that 18-to-49 age category compared with the same period last year, CBS lost 239,000 (8 percent), ABC lost 681,000 (21 percent) and Fox lost 709,000 (20 percent).

In the last few weeks, new viewership lows for network series have been recorded nightly among 18- to 49-year-olds, the group that still commands the highest advertising prices.

The declines have not discriminated. The bad news has been the same for hits, like ABC’s “Modern Family,” which had its lowest rating for the season (4.0 or about 5.2 million viewers) and less popular shows, like NBC’s “Community,” which descended to 1.3 (about 1.7 million viewers). Several other shows, like “Glee” and “Touch” on Fox, and “Missing” and “Suburgatory” on ABC, all hit their lowest ratings ever last week.

The losses could not have come at a worse time for the networks, which are about to enter the television upfronts, the traditional season when advertising dollars are committed for the fall season.

“These numbers are going to affect the upfronts,” said Brad Adgate, the senior vice president for research at Horizon Media. “These numbers represent billions of dollars in sales.”

Though there seems to be no one reason for the decline, many executives say they are concerned that long-term changes in watching habits are taking a significant toll on viewership.

The broadest explanation is the time of year. Each spring, viewership tends to decline because of factors like daylight saving time, which hurts the 8 p.m. shows especially because outdoor light discourages indoor viewing. And many television research executives said “nice weather” this spring may be encouraging people to spend more time outdoors.

But the clock changes every year without such a severe decline. As Mr. Adgate asked, “How many more people can be out smelling the roses this year?”

Network television, of course, is accustomed to being down. As noted by Michael Nathanson, the United States media analyst for Nomura Securities, the live ratings for network programs (that is, the ratings for people who watch shows when they are first broadcast) have declined for 14 straight quarters.

In the past, the network drop usually meant a bonanza for cable networks, which inherited those viewers. But over the same four weeks beginning in March, cable networks combined lost an average of 409,000 viewers, about 2 percent.

And yet, overall television viewing is flat this spring, according to Nielsen research. That means viewers are using their television sets just as much this year as last year.

So bring on the detectives. What is going on here?

Mr. Nathanson suggested one obvious suspect in the overall ratings decline has been the steep ratings drop for “American Idol” this season — more than 30 percent. He contends the struggles of “Idol” have had a disproportionate impact on the overall ratings for the live viewing of prime-time shows. “Idol,” which is a competition show broadcast live, “has in the past gotten you into the live ecosystem,” he said.

Many millions of people watched television live on “Idol” nights, he said, and with “Idol” declining, fewer people have the incentive to sit in front of the television on those nights.

Another explanation behind the steep decline in network shows is the way networks now parcel out episodes of their more popular offerings. Around March, they begin inserting strings of repeat, which, more than ever, viewers avoid. Jay Sures, a partner in the United Talent Agency, said his company’s research found that “the disruption of the ordered pattern of episodes is a big issue.”

Viewers can go weeks without seeing a new episode of a favorite like the comedy “Modern Family,” he noted, and they do not know when a new episode will be shown. Last week a new episode of “Modern Family” scored the lowest rating of the season.

At the same time, viewers who scheduled the show on a “season’s pass” on a DVR will learn later that they have a new episode to watch — and when they do, they will not be watching something else on live television. And that, many television executives say, may indicate a fundamental shift in how viewers consume television programming. They no longer watch nearly as much of it while it is broadcast.

Mr. Nathanson said the playback of prerecorded shows is so extensive, no show can be judged fairly by how it fares on its first night. He cited Nielsen figures that indicate recorded playback of shows would average a 4.7 rating in the 18-to-49 category for those four weeks this spring. That would make playback the second-highest-rated show on television for that period, just behind “American Idol” on Fox.

Two years ago at this time, Jeff Gaspin, then the head of entertainment at NBC, was trying to find television shows that would attract viewers and advertisers, and perhaps finally turn around the decade-long slump that afflicted the network.

Now Mr. Gaspin looks at the recent news of lower and lower ratings across the board and has one conclusion: “We are seeing the cumulative effect of nonlinear viewing.”

That is attributable, he said, to what he called the “built-up libraries” viewers can now watch. “I think we are at a tipping point in how people are going to watch shows,” he said.

Mr. Gaspin said that this year he and his 13-year-old son decided to try out the AMC series “The Walking Dead.” Hooked by the first two episodes, they set aside an hour at 9 each night to watch the first two years, hour by hour, which Mr. Gaspin had collected through every means available — some episodes from Netflix, some from iTunes, some recorded on the family DVR.

“We learned a new behavior,” Mr. Gaspin said. Finally they caught up to this season’s finale.

“We watched that live,” he said. “It was not nearly as good. The commercials broke the tension. We had watched the other episodes with blankets over our heads. I hate to say this to the AMC executives and everybody else in the business, but I will never watch ‘Walking Dead’ live again.”