In a sense, it's possible to measure the progress of the Zac Brown Band by the magnitude of their guest stars.

In 2010, they consolidated the breakthrough of 2008's Foundation by enlisting Jimmy Buffett and Alan Jackson for duets -- elders whose very presence suggested they were passing a torch (although, to be sure, Buffett has a far greater pull on Brown's sound than Jackson).

Two years later, it is the Zac Brown Band who occupy the power position, drafting in peers, not idols, to play alongside.

And it is a diverse batch: twee, twiddly Jason Mraz co-writes the sprightly opening cut, "Jump Right In," with Zac, Trombone Shorty colors "Overnight" with some New Orleans funk, and upcoming folk/blues troubadour Amos Lee sings on "Day That I Die," each guest representing a different field for the ZBB, each suggesting the range of this ever-evolving nominal country band.

And at this point, the Zac Brown Band would fit the grander stages of such worldly, knowing vaguely hippie enclaves as Bonnaroo better than they would a rocking country outlet somewhere in the Deep South.

But Southern they are, in sensibility and sound, reflecting not the dusty beer joints and cutthroat honky tonks of the middle of the 20th century but the sports bars and sandy beaches of the present, the kinds of places where the kin of the Allmans feel as Southern as the descendants of George Jones...and where a bearded soft rock crooner like Zac Brown is happy to make evident his debt to James Taylor.

Brown's sweeter side isn't hidden here but it's not quite as prominent as it's been in the past, either.

He has plenty of soft, crooning melodies but there's a bit of bluegrass and a bit of reggae, a little blues and a lot of rock.

Above anything else, Uncaged is a Zac Brown Band album, one that emphasizes the range of this quintet and its elastic interplay.

It is the sound of a band operating from a position of considerable strength: they're confident, assured, even playful, having fun bending the rules and blurring boundaries, eager to please but never pandering.

It's the rare album that suggests how good the band would be in concert yet still sounds vibrant on record.