The problem with The Birds, the first circus and theatre collaboration at the National, is that the overall creation is far less than the sum of its parts. Poet Sean O'Brien's new adaptation of Aristophanes' Greek classic, directed by Kathryn Hunter, is a combined effort with Mamaloucos Circus as part of the NT's Transformation season.

The Birds is in its original form is a political comedy about the futility of human power and how quickly a democracy can fall into a dictatorship. Two Athenians, renamed 'Pez' and 'Eck', escape from their war-torn land and take to the skies with their feathered friends, looking for a utopia where they can have the freedom of wings.

The project aims to combine theatrical narrative with circus-style visual spectacle. But somewhere along the way, Aristophanes' story has gone missing. Instead of soaring to great heights, the plot rather bumbles along and arrives at the next scene all of a sudden without any real development of character or event. O'Brien's translation is awkward and wordy, with modern references like 'Jimmy Choos' needlessly inserted in an apparent attempt to increase relevance.

Mamaloucos' circus elements, however, are truly spectacular, their aerial feats (choreographed by Matt Costain) on the trapeze and trampoline well deserving the applause they regularly receive. The chorus all quite believably have the gift of flight - swooping down over the audience on huge lengths of rope.

Each chorus member plays a different bird and, thanks too to Liz Cooke's designs as well as the company's physical performances, each - from the magnificent eagle (George Tsonev) through to a green-headed duck (James Roberts) and Franky Mwangi's fine sparrow - are carefully detailed, providing a visual feast of activity.

But the fact that there's always something to look at doesn't hide a lack of dramatic drive. Marcello Magni's Pez remains somewhat earthbound. When he takes a turn for a worse, a bird-eating dictator sort of turn, it's just not very convincing: portray Pez too much a fool and he's trapped as one. Eck, played by Hayley Carmichael, fares better, particularly during an impassioned speech, about the impotence of democracy, that stands out as the only point at which the text can be properly heard.

By all means, see The Birds to appreciate some consummate circus skills, but don't go expecting a theatrical coup. You won't break any new ground simply by taking to the air - and Mamaloucos certainly doesn't succeed in doing so here.

- Sarah Beaumont (reviewed at the NT Lyttelton)