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.