Svend Nielsen's (born 1937) Sommerfugledalen (Butterfly Valley), a requiem for 12 solo singers, is a setting of a 15-sonnet cycle by Danish poet Inger Christensen. The collection of poems, published in 1991, has already become a classic in Danish literature and has inspired numerous musical settings. Nielsen's version makes extraordinary demands on the unaccompanied vocal ensemble, requiring close chromatic harmonies, rhythmic complexity, and extended vocal techniques. The effect, however, is not one of cerebral complexity, but of the kind of fecund diversity found in nature -- unpredictable, evolving organically rather than by predetermined structure. Most of the settings have a luminous delicacy that's strongly evocative of the butterfly imagery of the poetry. The harmonic language is not exactly tonal, but uses harmonies that are largely triadic, and its melodic lines are usually lyrical. Christensen's structuring of the cycle is ingenious; the last sonnet consists of the first lines of each of the preceding 14 sonnets. Nielsen follows her pattern, with the final sonnet consisting of the music for the beginning of the earlier sonnets, and the effect, far from sounding contrived or disjunct, is surprisingly unified, and makes a tender and deeply moving conclusion to the cycle. Ars Nova Copenhagen, for whom the piece was written, sings with pure tone, precision, and nuanced flexibility. The CD concludes with Christensen reading the sonnets with measured dignity. Dacapo's sound is natural and spacious.