What American viewers got from the recent 10-hour TV mini-series "The Thorn Birds" was the video equivalent of a good read--a good watch. What the ABC television network got was an unexpectedly huge smash hit, a veritable one-show bonanza that earned Nielsen rating shares nearly 20 points higher than even industry savants predicted. Naturally this has network nabobs scratching their heads and asking a grateful, "But why?"

Whatever the reason for the strong "Thorn Birds" showing, its success, and that of "The Winds of War" a mere seven weeks earlier, all but guarantee a new wave of mini-series madness, like the kind that followed the still-record-holding success of "Roots" in 1977 (indeed, the three networks among them have 15 on the drawing boards for next season alone). This time, the trend may be more frantic than before, because the networks, stupefied and spooked by the economic uncertainties of the new video age, see in these blockbuster TV serials that which they most fervently desire: powerhouse programming that can draw viewers away from pay TV and cable TV and back to Daddy Networks, as both "Winds" and "Birds" did.

In fact, NBC and CBS may be nearly as cheered by ABC's success as ABC was, because the number of Homes Using Television (the closely watched "HUT" levels) went up during the weeks of "Winds" and "Birds" and that, in Madison Avenue eyes, reaffirms network dominance in American home entertainment. Networks now are a little like dinosaurs who've heard all this gossip about their forthcoming extinction and, panicky at the thought, run wild through the thicket, their walnut-sized brains searching desperately for schemes with which to postpone the Ice Age--or at least get themselves cozy shelter for chilly scenes of winter ahead.

Network executives are tired of reading that "the network era" is kaput and that cable and pay TV are the waves of the future. They also aren't cheered by the fact that their shares of the total viewing audience have been declining steadily for five years. Successes like "Winds" and "Birds" bolster network morale, get viewers back into the habit of network viewing, and act as at least temporary tide-stemmers.

Everybody knew "The Winds of War" would be a blockbuster, but hardly anybody could have foreseen that "Thorn Birds" would not only be a hit but actually earn better ratings than "Winds of War" did--the best ratings of any mini-series since the historic "Roots." Soon after "Winds" had copped second place among the all-time highest rated mini-series, "Birds" elbowed it right out and took the spot for itself.

Among those pleasantly surprised by this development was David L. Wolper, the executive producer of "The Thorn Birds" and of "Roots" and of five other mini-series, most of them successful. Indeed, Wolper says he produced the first network mini-series ever, a three-night presentation of "The Rise and Fall of the Third Reich" on ABC in 1967. Naturally, he is elated with the figures on his latest production. "The network sold 'The Thorn Birds' on a 40 share," says Wolper; that means ABC told advertisers they could expect to reach 40 percent of those watching television during the hours "Thorn Birds" was on. Wolper says the mini-series staff at ABC privately predicted a 45 share, he optimistically predicted 50, but the final tally was a phenomenal 59 share.

That translates into roughly 35 million American homes glued to the story of how a priest in Australia violated his vow of celibacy with a beautiful girl he first met when she was an enchanting child on a huge sheep ranch.

"Winds of War" concerned an American experience about which millions of living people have vivid firsthand or secondhand memories: the entry of this country into World War II. Its appeal was easy to understand. "Thorn Birds," on the other hand, was set almost entirely in Australia, foreign terrain to U.S. viewers, and dealt at great length with life on a sheep station, which is not exactly next-door stuff. Perhaps most impressively of all, there wasn't a Nazi to be seen in it anywhere. Nazis are generally worth 10 or 15 share points all by themselves.

That "Thorn Birds" nevertheless scored better ratings than "Winds" is encouraging in one respect: "Thorn Birds" was, as filmmaking and storytelling, far more sophisticated and better-acted. Visually, it was luminous. But quality has never been a guarantee of success in commercial TV. There have to be other reasons why "Thorn Birds" took off.

The Rev. Gilbert Hartke, founder of the Catholic University drama department here, thinks the production succeeded on notoriety. "It's just like 'Arrowsmith' years ago," he says. That was about a minister in the same situation. There's a prurient interest. People find this very titillating. It's like going behind a barn and reading a dirty book." The U.S. Catholic Conference protested the scheduling of the mini-series during Holy Week, but such protests may have contributed to an aura of forbidden fruit that encouraged viewers to tune in.

Certainly the sexiness of the show should not be overlooked as a key ingredient in its extreme popularity. "Thorn Birds" went a little further in its erotic depictions than previous TV films have gone. An executive at a rival network, who watched the show faithfully, said last week in New York, "It's the most sensual thing I've ever seen on television."

Wolper's own theories to explain the success of "Thorn Birds" have little to do with prurience. "I feel two things are responsible," he says from Hollywood. "One, the subject matter in the book is very deeply ingrained in women in this country. This was a very successful women's book--not just a throwaway kind, but deeply so. Second, we cast this a certain way. We could have used TV stars, people like Lee Majors and Cheryl Ladd--not to denigrate them--but we didn't. It was a soap opera, but we cast it classy, so it would elevate it. And as a result, we got what's known as the 'light television audience' to tune in, people who do not regularly watch television. We got the regular watchers, too, but in order to get over a 50 share, you have to get those light viewers."

Starting the mini-series on a Sunday night was a good idea, Wolper thinks, because men had been controlling the TV set all day--it was a heavy sports Sunday, as most are--and so when evening came, women took control. "The women said, 'I want to watch that damn thing,' " Wolper theorizes.

A woman in the top executive echelons of ABC says, however, that while she agrees "Thorn Birds" was a women's picture, that's not just because it was "romantic" in nature. "There were a lot of subthemes in there that had nothing to do with 'romance' but had a lot to say about relationships and people and things that women know more about than men do," she says. And very convincingly, too.

Wolper says that a successful mini-series has to fall into at least one of three categories: It must be based on a big best seller, it must deal with a great historical event, or it must deal with a momentous sociological occurrence. "Roots," Wolper says, had all three elements. "Winds of War" had two. "Thorn Birds" had only one, but it took it all the way to the bank. The First National Bank of Beverly Hills, as it were.

"A good story does not a mini-series make," Wolper says. "It has to fall into one of those three categories."

There are other factors in the success of "Thorn Birds," Wolper says. One is timing; the scheduling of the series during Holy Week may have irked the clergy, but putting the program on the air during nights when both Christians and Jews were less likely to be going out and more likely to be staying home was either a brilliant gambit or a lucky coincidence. Wolper credits the hard-driving ABC promotional machinery, which he thinks is the best in television, for making America "ThornBirds"-conscious. "We were able to hit the people over the head with a 2-by-4 to get their attention," he says proudly.

Wolper also says the excessive length of "Thorn Birds" (the $21 million film was initially to be eight air hours long, then grew to 10) contributed to its popularity. Three directors of theatrical films--Arthur Hiller, Herbert Ross, and Australia's Peter Weir--tried to develop a movie version of the Colleen McCullough novel, Wolper says, but all found they couldn't tell the story in two or 2 1/2 hours.

"You couldn't do it," Wolper says, "because if the priest fell in the first hour, it would have felt sleazy. You had to build up to it. He didn't fall until the seventh hour of our show, after we had built up great expectation in the audience. In love stories, in spite of the temptation to move things faster for the television audience, who are used to higher-paced viewing, it's sometimes better to take your time. 'Thorn Birds' required a leisurely pace. We decided to play against the television thing so you got all the juice."

Of the 15 mini-series Wolper says the networks have scheduled for next season, one is Wolper's own, long-delayed, "Mystic Warrior," a tale of American Indians formerly known as "Hanta Yo" and the object of much criticism from Indian leaders, now apparently placated. Wolper is himself concerned that if the networks overdo the mini-series ploy, it will dissipate the allure of the form. "After 'Roots,' everybody went mini-series crazy," Wolper groans. His "one fear" about "Thorn Birds" was that the TV audience would be "exhausted" after 18 hours of "Winds of War" and not up for another long spell in front of the set.

Of "Winds," when asked for his opinion, Wolper says diplomatically, "I liked the history. I wasn't thrilled with the story part."

Among the network executives who would not make themselves available for comment on "Thorn Birds" and the return of the mini-series were NBC's Brandon Tartikoff, ABC's Brandon Stoddard, and Harvey Shephard of CBS. But Wolper feels fairly certain a mini-series revival--if not renaissance--is at hand. The "novel for television," as ABC likes to call productions like "Thorn Birds," could help woo back to the networks some of the viewers who have been driven away by all the moronic weekly shows. But if in the rush to get serials on the air, they turn out to be moronic as well, the networks will have taken one step forward and one step back.

Whichever happens, minis to the max appear inevitably in the offing. "I'm having lunch with people from all three networks this week," Wolper chuckles. "That ought to tell you something right there."