Saint François d'Assise: A finale of wings and prayers

What could be a more heavenly way to draw an arts festival to a close than Messiaen's birdsong-filled opera, Saint François d'Assise?

When St Francis is miraculously granted a foretaste of celestial bliss in Olivier Messiaen's opera, Saint François d'Assise, at the closing concert of the International Festival, the composer's sister-in-law, Jeanne Loriod, will surely be there in spirit. Loriod, who died earlier this month, was to have played one of the three ondes martenot parts required by this colourful score. The festival's director, Brian McMaster, must be relieved that he hadn't decided to present the same composer's Fêtes des belles eaux instead. It requires no fewer than six of these exotic electronic keyboard instruments.

Without having to twist Pierre Boulez's arm (he's in Edinburgh the night before and can play the ondes martenot), McMaster has secured the services of Pascale Rousse-Lacordaire to join Dominique Kim and Valerie Hartmann-Claverie. The performance, to which Messiaen's now frail widow, Yvonne Loriod, has been invited, will be dedicated to the memory of her sister, Jeanne, and is likely to be a highly charged affair.

David Wilson-Johnson, who is St Francis, first sang the title role in semi-staged performances in London and France in 1988 to mark the composer's 80th birthday. Messiaen was around during the last few rehearsals for these performances – "a benign presence, though he was rather austere and didn't say much" – and, taking his saintly role to heart, Wilson-Johnson probably saved his life: "Messiaen was standing in an aisle, oblivious to the fact that a lorry was reversing into the studio where we were rehearsing. Everyone else had their backs to him except me, so I had to stop the proceedings abruptly so that he could be moved out of the way in time."

Having recently revisited the role in Brussels and New York, Wilson- Johnson enthuses about the team recreating the work in Edinburgh. "Reinbert de Leeuw had conducted every note of Messiaen's music except Saint François d'Assise, and the performances we did with the Netherlands Radio Philharmonic Orchestra were just fantastic. They were all on a mission. The principal percussionist spent six months memorising his part, and Heidi Grant Murphy [who sings the Angel] seems to embody the whole thing."

At around four hours, even with two half-hour intervals, the piece needs almost superhuman stamina and a lot of bananas, he says, to sustain his part. When he first sang it he prepared seriously. "I went to the gym for a year, lost a lot of weight and got fitter." The man whose nickname in the music business is Jumbo slimmed down for sainthood.

As you might expect from a composer whose music is distinguished by its hypnotic, timeless quality, his libretto is not dramatic in any conventional sense, presenting instead a series of isolated incidents or tableaux that meditate on aspects of the life of St Francis of Assisi. "Franciscan scenes in three acts and eight tableaux", as the composer described it, it's more of a ritual of religious mysticism or a sacred oratorio. Does Wilson-Johnson think that perhaps this is one occasion where an opera will work just as well in concert performance?

"It is operatic, but not in a stock, Verdian way, although it has some of the most beautiful Verdian lines you could ever want to sing. It's exciting because the vocal line is integrated into the harmony and that harmony is so rich that the music itself becomes music theatre. Perhaps because the conviction behind it is so strong, it's stunning in an almost Wagnerian way."

Messiaen, who initially doubted whether or not he had the ability to compose an opera at all, wanted audiences to be "as dazzled by it as I am". It contains, the composer explained, "virtually all the birdcalls that I've noted down in my life, all the colours of my chords, all my harmonic procedures, and even some surprising innovations". He had intended to include the episode of St Francis taming the wolf, but ducked out because of the difficulty of portraying an animal on stage. Instead, we have birds. One of the most complex and most gorgeous moments in the opera is the "Sermon to the Birds" in the sixth tableau.

"I think one's sense of time slows down," says Wilson-Johnson, "to accommodate the enormous spaces in the musical canvas. It's perhaps hard to imagine singing a sermon to the birds for 45 minutes, but the scene, though the longest in the opera, is astonishingly inventive.

"And when, at the end of the sermon, the birds strike up their chorus, it's completely exhilarating, both emotionally and lyrically."

Twitchers should listen out for the garden warbler, the capinera (the blackcap from Assisi), the Australian lyre-bird and, evoked by three horns, crotales, glockenspiel and vibra-phone, the oriole. Each character is represented by a particular birdcall – birds of Umbria and the Carceri, as well as birds of more distant lands. The Angel, for instance, is represented by the song of the gerygone, a New Caledonian warbler. As well as the turtle-dove and robin, there are birds that St Francis's brothers can't hear because, as he admits, "They sing in my dreams".

And then there are Messiaen's own religious convictions. How sympathetic do musicians need to be to his beliefs? (He composed only for the greater glory of God, describing his opera as "the infusion of grace into the soul of one of the greatest saints".) "I don't see how you can sing this piece and not be influenced by all that. It's like singing Christus in Bach's Passions. You don't have to believe but you can't fail to be moved. The music and the poetry take you on to a higher plane, as if in preparation for death, a drawing towards the divine presence. It's as if Mess-iaen's entire life was heading in that direction. After St Francis receives the stigmata, I sing "Lord! Music and poetry have led me to Thee: by image, by symbol, and in default of Truth... dazzle me forever by Thy excess of Truth..." And then I die. Bells ring. Everything disappears. The closer I get to the piece, the more difficult it is to sing it because it is deeply emotional and I mustn't blub."

Wilson-Johnson has been warned that St Francis must die on cue or his Resurrection, hymned in a glorious chorale by the Edinburgh Festival Chorus, will be accompanied by the most almighty racket. The Festival Fireworks have been timed to go off just above the Usher Hall a mere 30 minutes after Saint François d'Assise is due to end...

Usher Hall, Saturday 1 September, 5pm. Tickets: 0131-473 2000