You know something’s up when a studio spends millions of dollars on a picture’s ads and trailers, all the while desperately trying to keep you from finding out what it’s about. In this case, Fox played up the presence of popular comic actors Steve Martin, Jack Black and Owen Wilson, but played down — way, way down — the nature of the high jinks in which they were paid to engage.

The studio should have had more faith in the material and its target audience. The Big Year, though hardly destined for box-office domination, is an eccentric and frequently charming trifle, more entertaining by far than many of the movies its prolific stars have made.

The film’s shameful secret? It’s about birding. More specifically, it deals with the real-life annual contest of the title, in which hobbyists compete for the honor of having sighted the largest number of species within the geographical limits of North America in one 12-month period. There is no prize money. What is at stake, we learn, are jobs, bank accounts and marriages.

Martin plays Stu Preissler, a business tycoon who’s ready for a different sort of life but is constantly pulled back in by two underlings (Joel McHale and Kevin Pollak) who require his guidance as they negotiate a sensitive merger.

Black — are you sitting down? — snaps a long losing streak with his dialed-down performance as nuclear-power-plant engineer Brad Harris, whose wife has left him because of his ornithological obsession. He’s moved back in with his parents to regroup. Dad is played by Brian Dennehy, so you can imagine his reaction to the news that the 36-year-old plans to run off to spot birds for a year, armed only with a pair of binoculars and a pocketful of his folks’ credit cards.

Wilson costars as the Babe Ruth of the Big Year. He’s Kenny Bostick, whose record total of 732 species has made him both a legend and a target. He’s married to a beauty played by Rosamund Pike, who wants nothing more than for him to stay home and make a baby. But Bostick feels drawn to the great outdoors to protect his crown. In one scene, he literally leaves Pike mid-make-out session after receiving news of a rare bird’s location. Which seems closer to fantasy or science fiction than comedy to me. But I digress.

The movie is based on Mark Obmascik’s 2004 nonfiction book of the same name and directed by David Frankel (The Devil Wears Prada). The script by Howard Franklin is perhaps its most winning asset. He steers clear of the predictable (bird-poop gags, hobbyists portrayed as geeks), sets a tone of convivial rivalry, tosses in the occasional twist and keeps the funnier-than-average dialogue flying.

In addition to some spectacular nature cinematography and a parade of exotic winged creatures, the film offers subtle insights. For example, Stu, Brad, Kenny and their fellow plumage-seeking pilgrims at first come off as compulsives — borderline fanatics. One can envision them on a “Hoarders”-style exploitative reality show called “Birders.”

As we get to know them, though, it becomes clear they’re simply following their outsider bliss. They’re answering the same sort of inner call that mysteriously summons their feathered friends to migrate vast distances each year.

As I watched the film, I couldn’t help contemplating what a director such as Wes Anderson might have created from the same material. Frankel has not quite realized its oddball potential. Nonetheless, The Big Year is indisputably a far more rewarding 90 minutes than most are likely to suspect. And that includes most of the folks employed in the promotional division at Fox.