By Lisa Richwine and Chris Michaud
LOS ANGELES/NEW YORK (Reuters) - "Ender's Game," the futuristic story of children forced to fight an enemy race, battled to the top of box office charts in the United States and Canada, defeating senior citizen buddy comedy "Last Vegas" and animated turkey tale "Free Birds."
Based on a best-selling 1985 novel, "Ender's Game" collected $28 million in ticket sales over its first three days, according to studio estimates released on Sunday.
Raunchy comedy "Jackass Presents: Bad Grandpa," which topped last weekend's charts, dropped to second place with $20.5 million. "Last Vegas" debuted in the No. 3 spot, grabbing $16.5 million.
Elsewhere, Marvel action hero Thor hammered into international theaters for sequel "Thor: The Dark World," hauling in $109.4 million from Wednesday through Sunday, distributor Walt Disney Co said. The movie debuts in the United States and Canada on Friday.
"Ender's Game" stars Asa Butterfield as Ender Wiggin, a young boy who is singled out for his superior intellect and put through advanced warfare training. Harrison Ford plays Colonel Graff, the man who isolates Ender from his comrades and manipulates him into commanding a war.
The movie is the latest adaptation of a young adult novel brought to the big screen by studios hoping to start a new blockbuster franchise like "Twilight" and "The Hunger Games."
"Ender's Game" finished in line with projections from box office analysts, who forecast a $25 million to $30 million start. The movie was produced for $110 million by Lions Gate Entertainment's Summit Entertainment studio, Oddlot Entertainment and visual effects company Digital Domain.
More than half - 54 percent - of the audience was over age 25, making its appeal to younger moviegoers and potential for a sequel less clear.
"It looks like 'Ender's Game' is one-and-done," said Jeff Bock, senior box office analyst at Exhibitor Relations Co., noting that with next week's big domestic opening of "Thor" "there won't be much of the box office pie left for 'Ender's Game.'"
"Last Vegas" stars an A-list cast of Oscar-winning actors Robert De Niro, Michael Douglas, Morgan Freeman and Kevin Kline, playing four lifelong friends who reunite for a bachelor party. Mary Steenburgen plays a Vegas lounge singer in the movie, produced by CBS Films, a unit of CBS Corp, with a small budget of $28 million.
Studio officials said they were thrilled that the movie, which received an A-plus CinemaScore rating from people who had seen it, also exceeded its pre-opening tracking estimates.
"Free Birds," featuring the voices of Owen Wilson and Woody Harrelson, debuted in fourth place, ringing up $16.2 million at North American (U.S. and Canadian) theaters. The 3D movie tells the story of two turkeys that travel back in time to stop the birds from becoming the main course at Thanksgiving dinner.
The film, which cost $55 million to make, was co-financed by privately held Relativity Media and Reel FX Animation Studios.
Rounding out the charts, 3D space thriller "Gravity" pulled in $13.1 million. The movie's worldwide total through Sunday topped $425 million, according to Warner Bros., a unit of Time Warner Inc. "Bad Grandpa," released by Viacom Inc unit Paramount Pictures, brought its total to $62.1 million through two weekends.
"Escape Plan," the prison-escape thriller teaming Sylvester Stallone and Arnold Schwarzenegger, fell out of the top 10 to No. 11 in only its third week in release, with $2.3 million in receipts for a total of $21.6 million.
(Reporting by Lisa Richwine and Chris Michaud.; Editing by Christopher Wilson)