Bret McKenzie Sings With Kermit: 'Life's A Happy Song' From 'The Muppets' (VIDEO)

While "The Muppets" is all about reviving the spirit of the beloved Jim Henson franchise, it is also, on a behind-the-scenes level, bringing back together another quirky TV series.

During Kermit & Co.'s decade absence from singing and dancing on television, the New Zealand import "Flight of the Conchords" helped pick up the slack. A hipster hallucinogenic musical about two down and out musicians living on the lower east side of Manhattan (as well as the vast expanses of their imaginations), it only lasted two seasons, but two of its most important elements have joined up with the Jason Segel "Muppets" movie for ultimate musical happiness.

Along with James Bobin, who co-created the "Conchords" show for HBO, directing the film, Bret McKenzie, one half of the singing stars of the show, played a major part in creating its soundtrack. That includes its opening scene song, "Life's A Happy Song," which has largely become the theme for the film. A fun, bouncy tune that spouts optimism in every note, it's performed by Jason Segel and his Muppet pal Walter in the movie, with mass dance sequences as they traipse through their home of Smalltown, USA.

In a new feature about McKenzie in the NY Times, he talks trying to write music for the film, and the odd experience of trying to figure out which Muppets could, and which couldn't, sing.

On the yes list: bears, frogs and pigs. The no list: chickens, and other fowl.

"So I was like, 'Can we get the penguins to sing?'" McKenzie remembers about one song in particular that required an ensemble chorus. "And they'd say: 'No. Penguins don't sing.'"

Luckily, as noted, frogs can sing; Kermit, of course, is one of the Muppets' most famous vocalists. To celebrate his contribution, the NY Times had McKenzie sing his "Life's A Happy Song" with the head frog himself, creating a delightful duet you can see below.