(Adds details on Fox ratings, NBC's lineup)
May 12 (Reuters) - Fox TV's lineup for the upcoming television season will include a high-profile Batman show called "Gotham" while the network scales back on air time for its aging "American Idol."
The televised singing contest, now 13 years old and dragging on ratings for the network owned by Twenty-First Century Fox, will start the season at two nights a week during the auditions phase. But "it's quite likely it'll end up being a two-hour show on one night through most of its run," Fox Broadcasting Entertainment Chairman Kevin Reilly said on a conference call on Monday.
The move will cut Idol's hours to 37 from roughly 50 hours for the season. The show reigned for several years as the highest-rated program on U.S. television but has suffered a sharp audience decline, down 19 percent this season to 12.2 million viewers on average, according to Nielsen.
Overall, Fox ranks second among viewers 18 to 49, the group most prized by advertisers, and fourth in total viewers.
Fox, along with its broadcast rivals, is announcing its new shows in an annual spring presentation this week known as the upfronts, when networks try to persuade advertisers to buy billions of dollars worth of commercial time for the TV season that starts in the fall.
Fox's "Gotham," featuring Jada Pinkett Smith, follows a police officer in the world of Batman and charts the origins of the D.C. Comics superhero.
Last year, ABC brought a comic book movie franchise to TV with "Marvel's Agents of SHIELD," a spin off from "The Avengers" movie. The show debuted strong with 12 million viewers, but ratings fell and it now averages 5.9 million viewers, according to Nielsen.
The ABC show is centered around SHIELD Agent Phil Coulson but does not feature the superheroes that starred in "Avengers" on the big screen.
With Gotham, "the real selling point here is that these are actual franchise characters," Reilly said, such as Batman regulars, Bruce Wayne, the Penguin and the Riddler.
Rival NBC, during its pitch to advertisers on Monday, highlighted several dramas involving cops and espionage, including "Allegiance", a series about Russian spies, "Odyssey," which centers on a global military conspiracy, and "State of Affairs" starring movie actress Katherine Heigel as a CIA analyst.
The audience gave loud applause to a one-hour detective series called "The Mysteries of Laura" starring "Will & Grace" star Debra Messing and film actor Josh Lucas.
NBC is leading among 18- to 49-year olds, the group most prized by advertisers, for the first time since the 2003-04 season. The network is averaging 3.5 million primetime viewers in that age group, up 17 percent from a year ago, thanks to hits including singing competition "The Voice" and James Spader thriller "The Blacklist," according to Nielsen. It also received a boost from the Winter Olympics in February. (Reporting by Liana B. Baker; Editing by Lisa Von Ahn and Tom Brown)