Last year, the vocal group I Fagiolini made an impact on the summer festival scene with their staging of Monteverdi madrigals in cafes and restaurants around the country. This time they have joined forces with director John Fulljames and his ever-enterprising Opera Group for an evening devoted to musical birds.

The main event is a new opera from Ed Hughes, commissioned by the City of London festival and based on Aristophanes' satire on urban life and utopianism, The Birds. That is set alongside half an hour of smaller-scale avian antics to works by Clement Janequin, Thomas Ravenscroft and Per Norgard.

The performances are wonderfully accomplished. The sheer wit and charm of I Fagiolini's singing and acting tend to obscure the high musical standards involved in everything they do, whether it be presenting the narrative of Ravenscroft's extended partsong The Three Ravens, imitating taped birdsong in Norgard's D'Monstrantz Voogeli, or turning Janequin's Le Chant des Oyseaulx into a fanciful cameo of birdwatching life.

However, Hughes's opera relies on that charm and proficiency a bit too much. It all begins promisingly, with the 10 singers accompanied by just a cellist, a percussionist and a pre-recorded tape. The central character of Pitcher, the man who fetches up in the country of the birds and destabilises their peaceful society, is a spoken role taken by Glenville Hargreaves. But Hughes's music, at first a pleasant and unassuming mixture of rap-like patter and close-harmony ensembles, never goes anywhere; what you hear in the first 10 minutes is what you get for the next 45. And the self-conscious cleverness of Glyn Maxwell's relentlessly rhyming libretto soon starts to grate.

Fulljames's production is deft and economical and the Fagiolini singers have just the right light touch, but the whole package needs a bit more musical substance to make it into a really convincing evening.

· At the Town Hall, Cheltenham, on Monday. Box office: 01242 227979. Then touring.