It's a measure of the distance America has traveled that Bye Bye Birdie was mildly racy fun in 1960, and is merely mild fun today. Or maybe it's a measure of something different: a general decline, in our musical theater, of piquant individuality and inventiveness. Whatever the cause, Birdie as revived for the last staged concert of this year's Encores! series still offered Charles Strouse's catchy tunes, Lee Adams's workable lyrics, and the amusingly goofy situations of Michael Stewart's book, all now grown affectionately familiar, rather than weary, through 45 years of stock and amateur use. This time around, though, the show's tale of a just-drafted rock star and his mother-dominated manager causing multi-generational chaos in Middle America seemed bland rather than zesty. Jerry Zaks's pleasant, bright-colored production, populated with performers more amiable than memorable, captured little of the original's airy unexpectedness, and some of Casey Nicholaw's choreography seemed to be emulating Up With People rather than the zany, slightly kinky gymnastics of Gower Champion's original. Could it be that the average American preferred a spicier theatrical diet back then? Or are we just used to more heavily seasoned dishes today? Either way, this Birdie, though nourishing, was distinctly the salt-free, low-carb version.